pgsql: Clarify documentation for libpq's PQescapeBytea to mention the n

Clarify documentation for libpq's PQescapeBytea to mention the new hex
format.

Modify PQescapeStringConn() docs to be consisent with other escaping
functions.

Add mention problems with pre-9.0 versions of libpq using not understanding
bytea hex format to the 9.0 release notes.

Backpatch to 9.0 docs.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/888fda809239f2b4c7daa992f504c495f7ec7c92

Modified Files
--------------
doc/src/sgml/libpq.sgml | 46 ++++++++++++++++-------------------------
doc/src/sgml/release-9.0.sgml | 3 +-
2 files changed, 20 insertions(+), 29 deletions(-)