Here's a patch that makes issue the search scope.

It gets IDs of [issues that have a message with word_1] and intersects
that with IDs of [issues that have a message with word_n].

Then it joins that with the old message-scope results.

If one word is in issue title and the other[s] in messages, it still
won't find'em...

IOW, an awfully ugly, stupid way of coding this RFE:  I can't do SQL,
didn't even try to get clean Python. But it works :)