doc: Adjust branching and rebuilding strategy to match reality.

The amount of packages has more than tripled since this section was written.
Adjust the rebuild limits and cycle lengths based on current practices.

* doc/contributing.texi (Submitting Patches): Increase 'staging' rebuild limit
to 1800 packages, and adjust the cycle to six weeks.  Increase 'core-updates'
cycle to six months.

doc/contributing.texi

@@ -991,16 +991,16 @@ rebuilding induced, commits go to different branches, along these lines:
 @item 300 dependent packages or less
 @code{master} branch (non-disruptive changes).
 
-@item between 300 and 1,200 dependent packages
+@item between 300 and 1,800 dependent packages
 @code{staging} branch (non-disruptive changes).  This branch is intended
-to be merged in @code{master} every 3 weeks or so.  Topical changes
+to be merged in @code{master} every 6 weeks or so.  Topical changes
 (e.g., an update of the GNOME stack) can instead go to a specific branch
 (say, @code{gnome-updates}).
 
-@item more than 1,200 dependent packages
+@item more than 1,800 dependent packages
 @code{core-updates} branch (may include major and potentially disruptive
 changes).  This branch is intended to be merged in @code{master} every
-2.5 months or so.
+6 months or so.
 @end table
 
 All these branches are @uref{@value{SUBSTITUTE-SERVER},