patch 8.2.1978: making a mapping work in all modes is complicated

Problem:    Making a mapping work in all modes is complicated.
Solution:   Add the <Cmd> special key. (Yegappan Lakshmanan, closes #7282,
            closes 4784, based on patch by Bjorn Linse)
diff --git a/runtime/doc/autocmd.txt b/runtime/doc/autocmd.txt
index a540704..0a668c1 100644
--- a/runtime/doc/autocmd.txt
+++ b/runtime/doc/autocmd.txt
@@ -551,12 +551,15 @@
 							*CmdlineEnter*
 CmdlineEnter			After moving the cursor to the command line,
 				where the user can type a command or search
-				string.
+				string; including non-interactive use of ":"
+				in a mapping, but not when using |<Cmd>|.
 				<afile> is set to a single character,
 				indicating the type of command-line.
 				|cmdwin-char|
 							*CmdlineLeave*
-CmdlineLeave			Before leaving the command line.
+CmdlineLeave			Before leaving the command line; including
+				non-interactive use of ":" in a mapping, but
+				not when using |<Cmd>|.
 				Also when abandoning the command line, after
 				typing CTRL-C or <Esc>.
 				When the commands result in an error the