man: improve sysroot.readonly docs

man: improve sysroot.readonly docs

The explanation of sysroot.readonly is a little confusing - we say
that "everything else is mounted read-only" but it's perhaps clearer
to say /sysroot is mounted read-only.

Also note that read-only is the default with composefs.

Finally, document the option in ostree.repo-config even though it is
now considered legacy - as of commit 22b8e4f9 (#2930) - it is still
commonly seen in repo configs, so users will look to understand
what it means.