linux.4: clarify path translation

Try to be a little more explicit about the path translation mechanism
accessing /compat/linux/<path> then falling back to /<path>.

As suggested by martin@lispworks.com, refer to the compat path
explicitly, and correct an existing grammaro.

PR: 277804
Reviewed by: fernape
Sponsored by: The FreeBSD Foundation

(cherry picked from commit f66e71fa78e16164339f7fd4791306fb30165581)
(cherry picked from commit d1daec3d358eb5aaa38fa7c95fbfa330c46a69a1)