doc: note the use of LC_COLLATE in comm, join and uniq

doc: note the use of LC_COLLATE in comm, join and uniq

* doc/coreutils.texi (uniq invocation): Simplify the
text to remove the inconsequential mentioning of order,
while implying that LC_COLLATE can alter equality comparisons.
* src/comm.c (usage): Mention LC_COLLATE is significant.
* src/join.c (usage): Ditto.
* src/uniq.c (usage): Ditto. Also improve the summary.
Suggestion from Andries Brouwer