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

1 files changed, 10 insertions, 2 deletions

diff --git a/lib/libc/sys/poll.2 b/lib/libc/sys/poll.2
index dbd641f50bf7..feed43151624 100644
--- a/lib/libc/sys/poll.2
+++ b/lib/libc/sys/poll.2
@@ -28,7 +28,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd November 13, 2014
+.Dd November 1, 2018
 .Dt POLL 2
 .Os
 .Sh NAME
@@ -232,7 +232,15 @@ points outside the process's allocated address space.
 A signal was delivered before the time limit expired and
 before any of the selected events occurred.
 .It Bq Er EINVAL
-The specified time limit is invalid. One of its components is negative or too large.
+The specified time limit is invalid.
+One of its components is negative or too large.
+.It Bq Er EINVAL
+The number of pollfd structures specified by
+.Fa nfds
+exceeds the system tunable
+.Va kern.maxfilesperproc
+and
+.Dv FD_SETSIZE .
 .El
 .Sh SEE ALSO
 .Xr accept 2 ,