manual: Clarify that abbreviations of long options are allowed

The man page and code comments clearly state that abbreviations of long
option names are recognized correctly as long as they are unique.
Document this fact in the glibc manual as well.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Andreas Schwab <schwab@linux-m68k.org>
(cherry picked from commit db1efe02)