man: describe interaction with ProtectHome=/InaccessiblePaths= in BindMount=

man: describe interaction with ProtectHome=/InaccessiblePaths= in BindMount=

https://github.com/systemd/systemd/issues/7153#issuecomment-485252308

Apparently this is still confusing for people.

Longer-term, I think we should just make BindMount= automatically "upgrade"
(or "downgrade", depending on how you look at this), any InaccessiblePath=
mountpoints to "tmpfs". I don't see much point in forcing users to remember
this interaction. But let's at least document the status quo, we can always
update the docs if the code changes.