Document host key cross-certification.

doc/config.but

@@ -1747,7 +1747,7 @@ arbitrary port (say, \cw{localhost} port 10022) were forwarded to a
 second machine's SSH port (say, \cw{foovax} port 22), and then
 started a second PuTTY connecting to the forwarded port.
 
-In normal usage, the second PuTTY will access the host key cache
+In normal usage, the second PuTTY will access the \i{host key cache}
 under the host name and port it actually connected to (i.e.
 \cw{localhost} port 10022 in this example). Using the logical host
 name option, however, you can configure the second PuTTY to cache
@@ -2531,8 +2531,8 @@ If this box contains at least one host key or fingerprint when PuTTY
 makes an SSH connection, then PuTTY's automated host key management is
 completely bypassed: the connection will be permitted if and only if
 the host key presented by the server is one of the keys listed in this
-box, and the host key store in the Registry will be neither read
-\e{nor written}.
+box, and the \I{host key cache}host key store in the Registry will be
+neither read \e{nor written}, unless you explicitly do so.
 
 If the box is empty (as it usually is), then PuTTY's automated host
 key management will work as normal.

doc/gs.but

@@ -77,13 +77,13 @@ server and it sends you a different host key from the one you were
 expecting, PuTTY can warn you that the server may have been switched
 and that a spoofing attack might be in progress.
 
-PuTTY records the host key for each server you connect to, in the
-Windows \i{Registry}. Every time you connect to a server, it checks
-that the host key presented by the server is the same host key as it
-was the last time you connected. If it is not, you will see a
-warning, and you will have the chance to abandon your connection
-before you type any private information (such as a password) into
-it.
+PuTTY \I{host key cache}records the host key for each server you
+connect to, in the Windows \i{Registry}. Every time you connect to a
+server, it checks that the host key presented by the server is the
+same host key as it was the last time you connected. If it is not,
+you will see a warning, and you will have the chance to abandon your
+connection before you type any private information (such as a
+password) into it.
 
 However, when you connect to a server you have not connected to
 before, PuTTY has no way of telling whether the host key is the
@@ -97,7 +97,7 @@ network users are on the same side and spoofing attacks are
 unlikely, so you might choose to trust the key without checking it.
 If you are connecting across a hostile network (such as the
 Internet), you should check with your system administrator, perhaps
-by telephone or in person. (Some modern servers have more than one
+by telephone or in person. (Many servers have more than one
 host key. If the system administrator sends you more than one
 \I{host key fingerprint}fingerprint, you should make sure the one
 PuTTY shows you is on the list, but it doesn't matter which one it is.)

doc/index.but

@@ -850,7 +850,8 @@ saved sessions from
 
 \IM{logical host name} logical host name
 \IM{logical host name} host name, logical
-\IM{logical host name} host key, caching policy
+
+\IM{host key cache}{host key caching policy} host key caching policy
 
 \IM{web browsers} web browser
 

doc/using.but

@@ -201,6 +201,28 @@ resets associated timers and counters). For more information about
 repeat key exchanges, see \k{config-ssh-kex-rekey}.
 }
 
+\b \I{host key cache}Cache new host key type
+
+\lcont{
+Only available in SSH-2. This submenu appears only if the server has
+host keys of a type that PuTTY doesn't already have cached, and so
+won't use. Selecting a key here will allow PuTTY to use that key now
+and in future: PuTTY will do key here will cause a fresh key-exchange
+with the selected key, and immediately add that key to PuTTY's
+permanent cache (relying on the host key used at the start of the
+connection to cross-certify the new key). That key will be used for
+the rest of the current session; it may not actually be used for
+future sessions.
+
+Normally, PuTTY will carry on using a host key it already knows, even
+if the server offers key formats that PuTTY would otherwise prefer,
+to avoid host key prompts. As a result, if you've been using a server
+for some years, you may still be using an older key than a new user
+would use, due to server upgrades in the meantime. The SSH protocol
+unfortunately does not have organised facilities for host key migration
+and rollover, but this allows you to manually upgrade.
+}
+
 \b \I{Break, SSH special command}Break
 
 \lcont{
@@ -912,7 +934,7 @@ authentication} box in the Auth panel of the PuTTY configuration box
 \S2{using-cmdline-loghost} \i\c{-loghost}: specify a \i{logical host
 name}
 
-This option overrides PuTTY's normal SSH host key caching policy by
+This option overrides PuTTY's normal SSH \i{host key caching policy} by
 telling it the name of the host you expect your connection to end up
 at (in cases where this differs from the location PuTTY thinks it's
 connecting to). It can be a plain host name, or a host name followed
@@ -922,7 +944,7 @@ on this.
 \S2{using-cmdline-hostkey} \i\c{-hostkey}: \I{manually configuring
 host keys}manually specify an expected host key
 
-This option overrides PuTTY's normal SSH host key caching policy by
+This option overrides PuTTY's normal SSH \i{host key caching policy} by
 telling it exactly what host key to expect, which can be useful if the
 normal automatic host key store in the Registry is unavailable. The
 argument to this option should be either a host key fingerprint, or an