pgsql: docs: clarify references to md5 hash and md5 crypt in pgcrypto

docs: clarify references to md5 hash and md5 crypt in pgcrypto docs

Backpatch to 9.3.X.

Suggestion from Richard Neill

Branch
------
REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/ac9558c2fd18faf4f107bb66ee03bad85593e287

Modified Files
--------------
doc/src/sgml/pgcrypto.sgml | 17 +++++++++++------
1 file changed, 11 insertions(+), 6 deletions(-)

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