pgsql: Code + docs review for escaping of option values (commit 11a020e

Code + docs review for escaping of option values (commit 11a020eb6).

Avoid memory leak from incorrect choice of how to free a StringInfo
(resetStringInfo doesn't do it). Now that pg_split_opts doesn't scribble
on the optstr, mark that as "const" for clarity. Attach the commentary in
protocol.sgml to the right place, and add documentation about the
user-visible effects of this change on postgres' -o option and libpq's
PGOPTIONS option.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/cbc8d65639344c390a1d1a7f646c186ff3ad8693

Modified Files
--------------
doc/src/sgml/libpq.sgml | 9 ++++++---
doc/src/sgml/protocol.sgml | 12 ++++++------
doc/src/sgml/ref/postgres-ref.sgml | 14 ++++++++++----
src/backend/utils/init/postinit.c | 11 ++++++-----
src/include/miscadmin.h | 2 +-
5 files changed, 29 insertions(+), 19 deletions(-)