As per <http://sourceware.org/ml/gdb/2005-11/msg00419.html>.

gdb/doc/ChangeLog:
2005-11-21  Jim Blandy  <jimb@redhat.com>

	* gdb.texinfo (Tracepoint Packets): Document restrictions on
	placement of 'R' actions in tracepoint action packets; document
	dependence of 'X' and 'M' actions on a preceding 'R' action for
	their registers.