man: Note semantics combining root.transient with composefs.enabled

It's all quite confusing having to reason about both the pre-composefs
ostree and the composefs version. But hopefully soon we more firmly
leave behind that first legacy.

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

man/ostree-prepare-root.xml

@@ -130,7 +130,8 @@ License along with this library. If not, see <https://www.gnu.org/licenses/>.
                    directory is *not* a symlink to <literal>/var/opt</literal>, but is just an empty directory.
                 </para>
                 <para>
-                   Note the <literal>/usr</literal> mount point remains read-only by default.  This option is independent of <literal>etc.transient</literal> and <literal>sysroot.readonly</literal>;
+                   If this option is not combined with <literal>composefs.enabled</literal>, then <literal>/usr</literal> mount point remains read-only by default.
+                   This option is independent of <literal>etc.transient</literal> and <literal>sysroot.readonly</literal>;
                    it is supported for example to have <literal>root.transient=true</literal> but <literal>etc.transient=false</literal> in which case changes to <literal>/etc</literal> continue
                    to persist across updates, with the default OSTree 3-way merge applied.
                 </para></listitem>