refactor: correct comments and assertions about mapping rhs <Nop> (#18821)

Also avoid referring to mappings as "keymaps" in commands and docs.

						*map_empty_rhs* *map-empty-rhs*
You can create an empty {rhs} by typing nothing after a single CTRL-V (you
have to type CTRL-V two times).  Unfortunately, you cannot do this in a vimrc
file.

src/nvim/normal.c

@@ -3917,7 +3917,7 @@ static void nv_regreplay(cmdarg_T *cap)
   }
 }
 
-/// Handle a ":" command and <Cmd> or Lua keymaps.
+/// Handle a ":" command and <Cmd> or Lua mappings.
 static void nv_colon(cmdarg_T *cap)
 {
   bool cmd_result;