rustdoc: treat allowed_through_unstable_modules as deprecation

This ensures `std::intrinsics::transmute` is deemphasized
in the search engine and other UI, by cleaning it into a deprecation
without propagating it through reexports when the parent module
is stable.

tests/rustdoc-js-std/transmute.js

@@ -1,3 +1,4 @@
+const FILTER_CRATE = "std";
 const EXPECTED = [
     {
         // Keep this test case identical to `transmute-fail`, except the
@@ -6,7 +7,7 @@ const EXPECTED = [
         'others': [
             { 'path': 'std::intrinsics::simd', 'name': 'simd_as' },
             { 'path': 'std::intrinsics::simd', 'name': 'simd_cast' },
-            { 'path': 'std::intrinsics', 'name': 'transmute' },
+            { 'path': 'std::mem', 'name': 'transmute' },
         ],
     },
 ];