r339343 (by allanjude):
Document that sendfile(2) can return ENOTCAPABLE
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)
While here, pet the mandoc linter, when dealing with the section that
describes valid values for flags.