runtime(doc): mention special case of i_CTRL-R_-

closes: #12947
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/insert.txt

@@ -127,7 +127,11 @@ CTRL-R {register}				*i_CTRL-R*
 			'/'	the last search pattern
 			':'	the last command-line
 			'.'	the last inserted text
+                                                       *i_CTRL-R_-*
 			'-'	the last small (less than a line) delete
+				register. This is repeatable using |.| since
+				it remembers the register to put instead of
+				the literal text to insert.
 							*i_CTRL-R_=*
 			'='	the expression register: you are prompted to
 				enter an expression (see |expression|)