run, override: Clarify the effect of --nofilesystem

There are two reasonable interpretations for --nofilesystem=home:
either it revokes a previous --filesystem=home (as in Flatpak 1.12.2 and
older versions), or it completely forbids access to the home directory
(as in Flatpak 1.12.3). Clarify the man pages to indicate that it only
revokes a previous --filesystem=home. This will hopefully reduce
mismatches between the design and what users expect to happen, as
in flatpak#4654.

A subsequent commit will introduce a way to get the Flatpak 1.12.3
behaviour in a way that is more backwards-compatible with Flatpak 1.12.2
and older versions.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 7bbeed2b87b84d6d94006e25418b7f89a7784fdb)