docs: document behavior of CHAR() comparisons with chars < space

Space trimming rather than space-padding causes unusual behavior, which
might not be standards-compliant.

Also remove recently-added now-redundant C comment.

src/backend/utils/adt/varchar.c

@@ -846,18 +846,6 @@ bpcharcmp(PG_FUNCTION_ARGS)
 				len2;
 	int			cmp;
 
-	/*
-	 * Trimming trailing spaces off of both strings can cause a string
-	 * with a character less than a space to compare greater than a
-	 * space-extended string, e.g. this returns false:
-	 *		SELECT E'ab\n'::CHAR(10) < E'ab '::CHAR(10);
-	 * even though '\n' is less than the space if CHAR(10) was
-	 * space-extended.  The correct solution would be to trim only
-	 * the longer string to be the same length of the shorter, if
-	 * possible, then do the comparison.  However, changing this
-	 * might break existing indexes, breaking binary upgrades.
-	 * For details, see http://www.postgresql.org/message-id/CAK+WP1xdmyswEehMuetNztM4H199Z1w9KWRHVMKzyyFM+hV=zA@mail.gmail.com
-	 */
 	len1 = bcTruelen(arg1);
 	len2 = bcTruelen(arg2);