patch 8.1.0020: cannot tell whether a register is executing or recording

Problem:    Cannot tell whether a register is being used for executing or
            recording.
Solution:   Add reg_executing() and reg_recording(). (Hirohito Higashi,
            closes #2745)  Rename the global variables for consistency.  Store
            the register name in reg_executing.
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 279dd20..f891f44 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1018,6 +1018,8 @@
 	getreg()		get contents of a register
 	getregtype()		get type of a register
 	setreg()		set contents and type of a register
+	reg_executing()		return the name of the register being executed
+	reg_recording()		return the name of the register being recorded
 
 	shiftwidth()		effective value of 'shiftwidth'