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: <16288.1479610770@sss.pgh.pa.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(-)