libstdc++: Update documentation comments for namespace rel_ops

libstdc++: Update documentation comments for namespace rel_ops

The comments in <bits/stl_relops.h> describe problems that were solved
years ago (for GCC 3.1). The comparison operators in <iterator> are no
longer ambiguous with the rel_ops ones, so the linked mailing list
thread and FAQ entry aren't relevant now. The reference to std_utility.h
is also outdated as it's just called utility now, both in the source
tree and when installed.

The use of rel_ops is still frowned upon though, so replace the
discussion of ambiguities within libstdc++ headers with adminition about
using rel_ops in user code.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/stl_relops.h: Update documentation comments.