docs: Specify when branch points happen

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7149>

docs/releasing.rst

@@ -35,6 +35,8 @@ Feature releases
 ----------------
 
 -  Available approximately every three months.
+-  Feature releases are branched on or around the second Wednesday of
+   January, April, July, and October.
 -  Initial time plan available 2-4 weeks before the planned branchpoint
    (rc1) on the mesa-announce@ mailing list.
 -  Typically, the final release will happen after 4 candidates.