Add completion for empty 'defsearch'

vimperator/locale/en-US/options.txt

@@ -247,6 +247,9 @@ This means, it you set 'defsearch' to "youtube", then [c]:open arnold
 schwarzenegger[c] will be exactly the same as [c]:open youtube arnold
 schwarzenegger[c]. Therefore, you need to add a keyword or search engine
 "youtube" first.
+
+If 'defsearch' is empty, then Firefox will always attempt to open the
+raw [[arg]].
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