rustdoc-search: simplify rules for generics and type params

This commit is a response to feedback on the displayed type
signatures results, by making generics act stricter.

Generics are tightened by making order significant. This means
`Vec<Allocator>` now matches only with a true vector of allocators,
instead of matching the second type param. It also makes unboxing
within generics stricter, so `Result<A, B>` only matches if `B`
is in the error type and `A` is in the success type. The top level
of the function search is unaffected.

Find the discussion on:

* <449965149>
* <https://github.com/rust-lang/rust/pull/124544#issuecomment-2204272265>
* <476841363>

compiler/rustc_span/src/symbol.rs

@@ -1762,6 +1762,7 @@ symbols! {
         saturating_add,
         saturating_div,
         saturating_sub,
+        search_unbox,
         select_unpredictable,
         self_in_typedefs,
         self_struct_ctor,