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Correct out-of-date text that said the "default" collation is always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