Fix documentation and comments on what happens after GSS rejection

Fix documentation and comments on what happens after GSS rejection

The paragraph in the docs and the comment applied to
sslnegotiaton=direct, but not sslnegotiation=requiredirect. In
'requiredirect' mode, negotiated SSL is never used. Move the paragraph
in the docs under the description of 'direct' mode, and rephrase it.

Also the comment's reference to reusing a plaintext connection was
bogus. Authentication failure in plaintext mode only happens after
sending the startup packet, so the connection cannot be reused.

Reported-by: Jacob Champion
Discussion: https://www.postgresql.org/message-id/CAOYmi+=sj+1uydS0NR4nYzw-LRWp3Q-s5speBug5UCLSPMbvGA@mail.gmail.com