patch 8.1.2221: cannot filter :disp output

Problem:    Cannot filter :disp output.
Solution:   Support filtereing :disp output. (Andi Massimino, closes #5117)

runtime/doc/various.txt

@@ -575,6 +575,8 @@ N  *+X11*		Unix only: can restore window title |X11|
 			   |:marks|      - filter by text in the current file,
 					   or file name for other files
 			   |:oldfiles|   - filter by file name
+			   |:registers|  - filter by register contents
+					   (does not work multi-line)
 			   |:set|        - filter by variable name
 
 			Only normal messages are filtered, error messages are