ENH: added to splitCyclic.txt changes file.

doc/changes/splitCyclic.txt

@@ -104,6 +104,14 @@ type 'processorCyclic'.
 
 - processor patches use overlapping communication using a different message
 tag. This maps straight through into the MPI message tag.
+See processorCyclicPolyPatch::tag(). This needs to be calculated the
+same on both sides so is calculated as
+        Pstream::nProcs()*max(myProcNo, neighbProcNo)
+      + min(myProcNo, neighbProcNo)
+which is
+- unique
+- commutative
+- does not interfere with the default tag (= 1)
 
 
 - when constructing a GeometricField from a dictionary it will explicitly