Annotate only_one comment with additional context

See Textualize#1490

src/textual/css/query.py

@@ -246,7 +246,7 @@ def only_one(
             # should *not* be anything there, so we *should* get an
             # IndexError. We *could* have just checked the length of the
             # query, but the idea here is to do the check as cheaply as
-            # possible.
+            # possible. "There can be only one!" -- Duncan McLeod
             _ = self.nodes[1]
             raise TooManyMatches(
                 "Call to only_one resulted in more than one matched node"