pgsql: Doc: improve documentation about composite-value usage.

Doc: improve documentation about composite-value usage.

Create a section specifically for the syntactic rules around whole-row
variable usage, such as expansion of "foo.*". This was previously
documented only haphazardly, with some critical info buried in
unexpected places like xfunc-sql-composite-functions. Per repeated
questions in different mailing lists.

Discussion: <16288(dot)1479610770(at)sss(dot)pgh(dot)pa(dot)us>

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/ff9730aa15d4f964c0f4bb3bc73b12a9e5312e9a

Modified Files
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doc/src/sgml/queries.sgml | 3 +-
doc/src/sgml/rowtypes.sgml | 214 +++++++++++++++++++++++++++++++++++++++++++--
doc/src/sgml/syntax.sgml | 16 ++--
doc/src/sgml/xfunc.sgml | 69 +++------------
4 files changed, 231 insertions(+), 71 deletions(-)