Doc fix

* src/keymap.c (Fdefine_prefix_command): Clarify doc string
slightly (bug#18092).

(cherry picked from commit 6b769c81)

src/keymap.c

@@ -1737,10 +1737,12 @@ bindings; see the description of `lookup-key' for more details about this.  */)
 
 DEFUN ("define-prefix-command", Fdefine_prefix_command, Sdefine_prefix_command, 1, 3, 0,
        doc: /* Define COMMAND as a prefix command.  COMMAND should be a symbol.
-A new sparse keymap is stored as COMMAND's function definition and its value.
-If a second optional argument MAPVAR is given, the map is stored as
-its value instead of as COMMAND's value; but COMMAND is still defined
-as a function.
+A new sparse keymap is stored as COMMAND's function definition and its
+value.
+This prepares COMMAND for use as a prefix key's binding.
+If a second optional argument MAPVAR is given, it should be a symbol.
+The map is then stored as MAPVAR's value instead of as COMMAND's
+value; but COMMAND is still defined as a function.
 The third optional argument NAME, if given, supplies a menu name
 string for the map.  This is required to use the keymap as a menu.
 This function returns COMMAND.  */)