GUI Pageant stopped using SSH identifiers for key types in fea08bb244,
but the docs were still referring to them.
As part of this, ensure that the term "NIST" is thoroughly
cross-referenced and indexed, since it now appears so prominently in
Pageant.
(While I'm there, reword the "it's OK that elliptic-curve keys are
smaller than RSA ones" note, as I kept tripping over the old wording.)