kern_poll: Restore explanatory comment removed in r177374

The comment isn't stale. The check is bogus in the sense that poll(2)
does not require pollfd entries to be unique in fd space, so there is no
reason there cannot be more pollfd entries than open or even allowed
fds. The check is mostly a seatbelt against accidental misuse or
abuse. FD_SETSIZE, while usually unrelated to poll, is used as an
arbitrary floor for systems with very low kern.maxfilesperproc.

Additionally, document this possible EINVAL condition in the poll.2
manual.

No functional change.

Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D17671