Add _FORTIFY_SOURCE implementation documentation [BZ #28998]

There have been multiple requests to provide more detail on how the
_FORTIFY_SOURCE macro works, so this patch adds a new node in the
Library Maintenance section that does this.  A lot of the description is
implementation detail, which is why I put this in the appendix and not
in the main documentation.

Resolves: BZ #28998.
Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Florian Weimer <fweimer@redhat.com>