Document cmdgen ability to read public-only keys.

(This was added in 3935cc3.)

doc/man-pg.but

@@ -21,8 +21,7 @@
 
 \c{puttygen} is a tool to generate and manipulate SSH public and
 private key pairs. It is part of the PuTTY suite, although it can
-also interoperate with the private key formats used by some other
-SSH clients.
+also interoperate with the key formats used by some other SSH clients.
 
 When you run \c{puttygen}, it does three things. Firstly, it either
 loads an existing key file (if you specified \e{keyfile}), or
@@ -45,10 +44,21 @@ The options to control this phase are:
 
 \dt \e{keyfile}
 
-\dd Specify a private key file to be loaded. This private key file can
-be in the (de facto standard) SSH-1 key format, or in PuTTY's SSH-2
-key format, or in either of the SSH-2 private key formats used by
-OpenSSH and ssh.com's implementation.
+\dd Specify a key file to be loaded.
+
+\lcont{
+
+Usually this will be a private key, which can be in the (de facto
+standard) SSH-1 key format, or in PuTTY's SSH-2 key format, or in
+either of the SSH-2 private key formats used by OpenSSH and
+ssh.com's implementation.
+
+You can also specify a file containing only a \e{public} key here.
+The operations you can do are limited to outputting another public
+key format or a fingerprint. Public keys can be in RFC 4716 or
+OpenSSH format, or the standard SSH-1 format.
+
+}
 
 \dt \cw{\-t} \e{keytype}