rustdoc search: for return-type based searches, deprioritize functions that also have that type in an argument

[lolbinarycat]

for example, searching the rustc docs for -> Context returns several clone methods.

what this type of search means is "how do i get values of this type", in which case clone is not very useful, and stuff like constructors should be shown first.

this would apply to both -> Type searches and also the "in return types" tab.

[lolbinarycat]

this is the case for any type implementing Clone whose cannonical constructor has many arguments

[lolbinarycat]

it is also frequently the case for error types without a constructor, such as AllocError.