pgsql: doc: Clean up some recently added PL/pgSQL documentation

doc: Clean up some recently added PL/pgSQL documentation

- Capitalize titles consistently.
- Fix some grammar.
- Group "Obtaining Information About an Error" under "Trapping Errors",
but make "Obtaining the Call Stack Context Information" its own
section, since it's not about errors.

Branch
------
REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/98c5fe5d0df8549972f46c324d9c6b18b4c4ecc0

Modified Files
--------------
doc/src/sgml/plpgsql.sgml | 39 ++++++++++++++-------------------------
1 file changed, 14 insertions(+), 25 deletions(-)

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