MFC r339343,r343444:

MFC r339343,r343444:

r339343 (by allanjude):

Document that sendfile(2) can return ENOTCAPABLE

PR:		232207

r343444:

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
Approved by:	jtl (mentor)
Differential Revision: https://reviews.freebsd.org/D19150