mmap(2): Document guard size for MAP_STACK and related EINVAL.

Based on submission by:	emaste
Reviewed by:	emaste, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D26894

lib/libc/sys/mmap.2

@@ -28,7 +28,7 @@
 .\"	@(#)mmap.2	8.4 (Berkeley) 5/11/95
 .\" $FreeBSD$
 .\"
-.Dd February 26, 2020
+.Dd October 21, 2020
 .Dt MMAP 2
 .Os
 .Sh NAME
@@ -351,6 +351,8 @@ Stacks created with
 automatically grow.
 Guards prevent inadvertent use of the regions into which those
 stacks can grow without requiring mapping the whole stack in advance.
+The size of the guard, in pages, is specified by sysctl
+.Dv security.bsd.stack_guard_page .
 .El
 .Pp
 The
@@ -452,6 +454,11 @@ or
 was specified.
 At least one of these flags must be included.
 .It Bq Er EINVAL
+.Dv MAP_STACK
+was specified and
+.Va len
+is less than or equal to the guard size.
+.It Bq Er EINVAL
 .Dv MAP_FIXED
 was specified and the
 .Fa addr