getitimer.2: Rewrite a few pieces to clarify some details

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
fcntl.2: The return value for F_SETPIPE_SZ is the pipe capacity

Reported-by: Tomi Salminen <tlsalmin@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>