realpath.3: Rework resolved_path==NULL discussion w.r.t. POSIX.1-200[18]

realpath.3: Rework resolved_path==NULL discussion w.r.t. POSIX.1-200[18]

Although the page already mentioned the resolved_path==NULL
feature, and that this feature was added in POSIX.1-2008, there
was still some crufty text in BUGS that hadn't been updated to
reflect the POSIX.1-2008 changes.

Also, some other minor wording and grammar fixes.

Reported-by: Reuben Thomas <rrt@femur.dyndns.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>