pgsql: Documentation update for Standard Collations.

Documentation update for Standard Collations.

Correct out-of-date text that said the "default" collation is always
based on LC_COLLATE and LC_CTYPE.

Also reformat into a list to make it easier to understand and compare
the available collations, and briefly document the stability
characteristics of each one.

Discussion: https://postgr.es/m/4a69d067374d2f6bfb66f5bfb2ab9a020493d49f.camel@j-davis.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/875e46a0a246e416b12a9debe084ede9d02f1b5d

Modified Files
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doc/src/sgml/charset.sgml | 72 +++++++++++++++++++++++++++++------------------
1 file changed, 45 insertions(+), 27 deletions(-)