Revert "manpages: Document the new details of --nofilesystem behaviour."

The new behaviour caused regressions in some situations that previously
worked, and will be reverted.

This reverts commit 4d11f77aa7fd3e64cfa80af89d92567ab9e8e6fa.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit dfe868d628d8bcc09e98e64b1f790a525dff9205)