pgsql: Doc: improve PREPARE documentation, cross-referencing to plan_ca

Doc: improve PREPARE documentation, cross-referencing to plan_cache_mode.

The behavior described in the PREPARE man page applies only for the
default plan_cache_mode setting, so explain that properly.  Rewrite
some of the text while I'm here.  Per suggestion from Bruce.

Discussion: https://postgr.es/m/20190930155505.GA21095@momjian.us

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ca0b3828504a51247607913e16f45c635ec168b6

Modified Files
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doc/src/sgml/config.sgml      | 30 ++++++++++-----------
doc/src/sgml/ref/prepare.sgml | 63 ++++++++++++++++++++++++-------------------
2 files changed, 50 insertions(+), 43 deletions(-)