docs/process: branching-checklist: Reword the section about Config.mk

Since at least Xen 4.12, the revision for external trees don't contain
the word "unstable". So explicitly list the *_REVISION variables that
need to be updated as part of the branching process.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>

----
    Changes in v2:
        * Explain what should be the tag format for each variables

docs/process/branching-checklist.txt

@@ -69,11 +69,12 @@ ov=4.0
     git add versions
     git commit -m "Branch for $v"
 
-Ensure references to qemu trees in xen.git's Config.mk are updated.
-Check this with
-    grep unstable Config.mk
-which should produce no output.  Replace as necessary.
-(There may well be none.)
+Ensure references to qemu trees and Mini-OS in xen.git's Config.mk are updated.
+The variables and there content should be:
+  * QEMU_UPSTREAM_REVISION: qemu-xen-X.Y.0
+  * QEMU_TRADITIONAL_REVISION: xen-X.Y.0
+  * MINIOS_UPSTREAM_REVISION: xen-RELEASE-X.Y.0
+Where X.Y is the release version (e.g. 4.17).
 
 Update newly diverging staging (unstable) according to
 release-technician-checklist.txt section re README etc.