pgsql: Clarify usage of clientcert authentication option.

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(dot)1410(dot)30407(at)wrigleys(dot)postgresql(dot)org>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/745513c70282180afd83c666e43bdb0b6fb8c688

Modified Files
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doc/src/sgml/client-auth.sgml | 25 ++++++++++++++++++++++---
doc/src/sgml/runtime.sgml | 29 +++++++++++++++++------------
2 files changed, 39 insertions(+), 15 deletions(-)