Index "MAC" a bit more thoroughly.

[originally from svn r7610]

doc/errors.but

@@ -201,7 +201,7 @@ Upgrade your server, or use the workarounds described in
 \k{config-ssh-bug-ignore1} and possibly \k{config-ssh-bug-plainpw1}.
 
 \H{errors-crc} \q{Incorrect \i{CRC} received on packet} or \q{Incorrect
-MAC received on packet}
+\i{MAC} received on packet}
 
 This error occurs when PuTTY decrypts an SSH packet and its checksum
 is not correct. This probably means something has gone wrong in the

doc/index.but

@@ -672,8 +672,8 @@ saved sessions from
 \IM{ignore message} SSH \q{ignore} messages
 \IM{ignore message} \q{ignore} messages, in SSH
 
-\IM{message authentication code} message authentication code
-\IM{message authentication code} MAC (message authentication code)
+\IM{message authentication code}{MAC} message authentication code (MAC)
+\IM{message authentication code}{MAC} MAC (message authentication code)
 
 \IM{signatures} signature
 \IM{signatures} digital signature