completion: when the cursor is not after a word fragment, say so

Instead of being entirely silent when ^] is hit after whitespace
or punctuation, report what is lacking -- similar to M-] saying
"Not a bracket" when the cursor is not sitting on a bracket.
This makes the ^] keystroke more discoverable.

src/text.c

@@ -3600,6 +3600,7 @@ void complete_a_word(void)
 
 	/* If there is no word fragment before the cursor, do nothing. */
 	if (start_of_shard == openfile->current_x) {
+		statusbar(_("No word fragment"));
 		pletion_line = NULL;
 		return;
 	}