Rewrite some comments with FIXMEs in them.

These are things where no fix was actually necessary in the code, but
the FIXME indicated that the comment itself was either in need of a
rewrite or removal.

unix/uxstore.c

@@ -282,14 +282,19 @@ void close_settings_w(settings_w *handle)
     sfree(handle);
 }
 
-/*
- * Reading settings, for the moment, is done by retrieving X
- * resources from the X display. When we introduce disk files, I
- * think what will happen is that the X resources will override
- * PuTTY's inbuilt defaults, but that the disk files will then
- * override those. This isn't optimal, but it's the best I can
- * immediately work out.
- * FIXME: the above comment is a bit out of date. Did it happen?
+/* ----------------------------------------------------------------------
+ * System for treating X resources as a fallback source of defaults,
+ * after data read from a saved-session disk file.
+ *
+ * The read_setting_* functions will call get_setting(key) as a
+ * fallback if the setting isn't in the file they loaded. That in turn
+ * will hand on to x_get_default, which the front end application
+ * provides, and which actually reads resources from the X server (if
+ * appropriate). In between, there's a tree234 of X-resource shaped
+ * settings living locally in this file: the front end can call
+ * provide_xrm_string() to insert a setting into this tree (typically
+ * in response to an -xrm command line option or similar), and those
+ * will override the actual X resources.
  */
 
 struct skeyval {
@@ -352,6 +357,11 @@ static const char *get_setting(const char *key)
     return x_get_default(key);
 }
 
+/* ----------------------------------------------------------------------
+ * Main code for reading settings from a disk file, calling the above
+ * get_setting() as a fallback if necessary.
+ */
+
 struct settings_r {
     tree234 *t;
 };