patch 8.1.2044: no easy way to process postponed work

Problem:    No easy way to process postponed work. (Paul Jolly)
Solution:   Add the SafeState autocommand event.

runtime/doc/autocmd.txt

@@ -355,6 +355,9 @@ Name			triggered by ~
 			when popup menu visible
 |TextYankPost|		after text has been yanked or deleted
 
+|SafeState|		nothing pending, going to wait for the user to type a
+			character
+
 |ColorSchemePre|	before loading a color scheme
 |ColorScheme|		after loading a color scheme
 
@@ -955,6 +958,27 @@ RemoteReply			When a reply from a Vim that functions as
 				Note that even if an autocommand is defined,
 				the reply should be read with |remote_read()|
 				to consume it.
+							*SafeState*
+SafeState			When nothing is pending, going to wait for the
+				user to type a character.
+				This will not be triggered when:
+				- an operator is pending
+				- a register was entered with "r
+				- halfway executing a command
+				- executing a mapping
+				- there is typeahead
+				- Insert mode completion is active
+				- Command line completion is active
+				You can use `mode()` to find out what state
+				Vim is in.  That may be:
+				- VIsual mode
+				- Normal mode
+				- Insert mode
+				- Command-line mode
+				Depending on what you want to do, you may also
+				check more with `state()`, e.g. whether the
+				screen was scrolled for messages.
+
 							*SessionLoadPost*
 SessionLoadPost			After loading the session file created using
 				the |:mksession| command.