doc: Uncomment and tweak P2P docs

Now that P2P support is enabled unconditionally, uncomment the
P2P-related parts of the man pages.

Closes: #1803
Approved by: alexlarsson

doc/flatpak-build-commit-from.xml

@@ -46,11 +46,11 @@
             contents (and most of the metadata) taken from another
             branch, either from another repo, or from another branch in
             the same repository.
-            <!-- FIXME: Uncomment this when enable-p2p is enabled unconditionally.
+        </para>
+        <para>
             The collection ID set on
             <arg choice="plain">DST-REPO</arg> (if set) will be used for the
             newly created commits.
-            -->
         </para>
         <para>
             This command is very useful when you want to maintain a branch