pgsql: Doc improvements for language tags and custom ICU collations.

Doc improvements for language tags and custom ICU collations.

Separate the documentation for language tags themselves from the
available collation settings which can be included in a language tag.

Include tables of the available options, more details about the
effects of each option, and additional examples.

Also include an explanation of the "levels" of textual features and
how they relate to collation.

Discussion: https://postgr.es/m/25787ec7-4c04-9a8a-d241-4dc9be0b1ba3@postgresql.org
Reviewed-by: Jonathan S. Katz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1e16af8ab5d7f307b66e496eff6ec573d9fd8eb0

Modified Files
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doc/src/sgml/charset.sgml | 683 ++++++++++++++++++++++++++++++++++++++--------
1 file changed, 562 insertions(+), 121 deletions(-)