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