pgsql: Fix documentation of regular expression character-entry escapes.

Fix documentation of regular expression character-entry escapes.

The docs claimed that \uhhhh would be interpreted as a Unicode value
regardless of the database encoding, but it's never been implemented
that way: \uhhhh and \xhhhh actually mean exactly the same thing, namely
the character that pg_mb2wchar translates to 0xhhhh. Moreover we were
falsely dismissive of the usefulness of Unicode code points above FFFF.
Fix that.

It's been like this for ages, so back-patch to all supported branches.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/dad7ea7e4dcc9ebdfb5480aa91cd1424135214e9

Modified Files
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doc/src/sgml/func.sgml | 21 +++++++++++++++++----
1 file changed, 17 insertions(+), 4 deletions(-)