pgsql: Assorted editing for collation documentation.

Assorted editing for collation documentation.

I made a pass over this to familiarize myself with the feature, and found
some things that could be improved.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a612b17120fc011cefcdec6948b1cc8543529d06

Modified Files
--------------
doc/src/sgml/catalogs.sgml             |   57 ++++++++++-------
doc/src/sgml/charset.sgml              |  108 +++++++++++++++++++-------------
doc/src/sgml/ref/create_collation.sgml |   39 ++++++------
doc/src/sgml/ref/drop_collation.sgml   |    2 +-
4 files changed, 118 insertions(+), 88 deletions(-)