Clarify usage of clientcert authentication option.

For some reason this option wasn't discussed at all in client-auth.sgml.
Document it there, and be more explicit about its relationship to the
"cert" authentication method.  Per gripe from Srikanth Venkatesh.

I failed to resist the temptation to do some minor wordsmithing in the
same area, too.

Discussion: <20160713110357.1410.30407@wrigleys.postgresql.org>