mention restriction for use with fix shake or fix rigid in fix gcmc docs

doc/src/fix_gcmc.txt

@@ -383,6 +383,9 @@ called.  Reneighboring is required.
 Can be run in parallel, but aspects of the GCMC part will not scale
 well in parallel. Only usable for 3D simulations.
 
+When using fix gcmc in combination with fix shake or fix rigid,
+only gcmc exchange moves are supported.
+
 Note that very lengthy simulations involving insertions/deletions of
 billions of gas molecules may run out of atom or molecule IDs and
 trigger an error, so it is better to run multiple shorter-duration