doc: Mention fatal_proc_unmounted limitations
Discovered this while trying to port programs that use the deleted
libprocps function look_up_our_self() which can be found with the
fatal_proc_unmounted() function.
While procps_pids_new() will allow you to specify any items you
care to think of, a subsequent call to fatal_proc_unmounted()
will only fill in the values found in /proc/self/stat.
Added a caveat to the procps_pids manpage pointing out this
limitation.
References:
https://salsa.debian.org/xorg-team/app/apitrace/-/blob/debian-unstable/lib/os/os_memory.hpp#L44
https://gitlab.com/-/snippets/
2377884
Signed-off-by: Craig Small <csmall@dropbear.xyz>