Document that sendfile will return an invalid value for sbytes if provided an invalid address

This is meant to clarify the fact that the system call will not fail
with -1/EFAULT, as one might expect, when reading the sendfile(2)
manpage today.

While here, pet the mandoc linter, when dealing with the section that
describes valid values for `flags`.

PR:	232210
MFC after:	2 weeks
Approved by:	emaste (mentor)
Reviewed by:	glebius, 0mp
Differential Revision: https://reviews.freebsd.org/D18949

lib/libc/sys/sendfile.2

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 12, 2018
+.Dd January 25, 2019
 .Dt SENDFILE 2
 .Os
 .Sh NAME
@@ -104,7 +104,7 @@ variable pointed to by
 The least significant 16 bits of
 .Fa flags
 argument is a bitmap of these values:
-.Bl -tag -offset indent
+.Bl -tag -offset indent -width "SF_USER_READAHEAD"
 .It Dv SF_NODISKIO
 This flag causes
 .Nm
@@ -403,3 +403,14 @@ The
 .Fx 11
 implementation was written by
 .An Gleb Smirnoff Aq Mt glebius@FreeBSD.org .
+.Sh BUGS
+The
+.Fn sendfile
+system call will not fail, i.e., return
+.Dv -1
+and set
+.Va errno
+to
+.Er EFAULT ,
+if provided an invalid address for
+.Fa sbytes .