rustdoc: add support for type filters in arguments and generics

This makes sense, since the search index has the information in it,
and it's more useful for function signature searches since a
function signature search's item type is, by definition, some type
of function (there's more than one, but not very many).

tests/rustdoc-js-std/parser-literal.js

@@ -16,11 +16,11 @@ const PARSED = [
                     generics: [],
                 },
             ],
+            typeFilter: -1,
         }],
         foundElems: 1,
         original: "R<P>",
         returned: [],
-        typeFilter: -1,
         userQuery: "r<p>",
         error: null,
     }