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/getchar.h

@@ -54,8 +54,8 @@ struct map_arguments {
 
   char_u *rhs;  /// The {rhs} of the mapping.
   size_t rhs_len;
-  LuaRef rhs_lua;  /// lua function as rhs
-  bool rhs_is_noop;  /// True when the {orig_rhs} is <nop>.
+  LuaRef rhs_lua;  /// lua function as {rhs}
+  bool rhs_is_noop;  /// True when the {rhs} should be <Nop>.
 
   char_u *orig_rhs;  /// The original text of the {rhs}.
   size_t orig_rhs_len;