pgsql: More documentation updates for LATERAL.

More documentation updates for LATERAL.

Extend xfunc.sgml's discussion of set-returning functions to show an
example of using LATERAL, and recommend that over putting SRFs in the
targetlist.

In passing, reword func.sgml's section on set-returning functions so
that it doesn't claim that the functions listed therein are all the
built-in set-returning functions. That hasn't been true for a long
time, and trying to make it so doesn't seem like it would be an
improvement. (Perhaps we should rename that section?)

Both per suggestions from Merlin Moncure.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/06310178ef54720bc4ed7bc85c887e6ebed24d83

Modified Files
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doc/src/sgml/func.sgml | 7 ++--
doc/src/sgml/xfunc.sgml | 77 ++++++++++++++++++++++++++++++++++++++--------
2 files changed, 67 insertions(+), 17 deletions(-)