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

doc/coreutils.texi

@@ -4406,8 +4406,8 @@ duplicate lines, perhaps you want to use @code{sort -u}.
 @xref{sort invocation}.
 
 @vindex LC_COLLATE
-Comparisons use the character collating sequence specified by the
-@env{LC_COLLATE} locale category.
+Comparisons honor the rules specified by the @env{LC_COLLATE}
+locale category.
 
 If no @var{output} file is specified, @command{uniq} writes to standard
 output.

src/comm.c

@@ -129,6 +129,10 @@ and column three contains lines common to both files.\n\
 "), stdout);
       fputs (HELP_OPTION_DESCRIPTION, stdout);
       fputs (VERSION_OPTION_DESCRIPTION, stdout);
+      fputs (_("\
+\n\
+Note, comparisons honor the rules specified by `LC_COLLATE'.\n\
+"), stdout);
       emit_bug_reporting_address ();
     }
   exit (status);

src/join.c

@@ -204,6 +204,7 @@ separated by CHAR.\n\
 \n\
 Important: FILE1 and FILE2 must be sorted on the join fields.\n\
 E.g., use `sort -k 1b,1' if `join' has no options.\n\
+Note, comparisons honor the rules specified by `LC_COLLATE'.\n\
 If the input is not sorted and some lines cannot be joined, a\n\
 warning message will be given.\n\
 "), stdout);

src/uniq.c

@@ -135,8 +135,10 @@ Usage: %s [OPTION]... [INPUT [OUTPUT]]\n\
 "),
 	      program_name);
       fputs (_("\
-Discard all but one of successive identical lines from INPUT (or\n\
-standard input), writing to OUTPUT (or standard output).\n\
+Filter adjacent matching lines from INPUT (or standard input),\n\
+writing to OUTPUT (or standard output).\n\
+\n\
+With no options, matching lines are merged to the first occurrence.\n\
 \n\
 "), stdout);
      fputs (_("\
@@ -170,6 +172,7 @@ characters.  Fields are skipped before chars.\n\
 \n\
 Note: 'uniq' does not detect repeated lines unless they are adjacent.\n\
 You may want to sort the input first, or use `sort -u' without `uniq'.\n\
+Also, comparisons honor the rules specified by `LC_COLLATE'.\n\
 "), stdout);
       emit_bug_reporting_address ();
     }