man-pages.7: wfix

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
getopt.3: Ensure that 'nsecs' is used

Reported-by: Jonny Grant <jg@jguk.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
man-pages.7: Clarify that SEE ALSO entries may refer to pages from other projects

Reported-by: Laurent Georget <laurent.georget@supelec.fr>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>