Add "experimental" features to docs.rs (#2605)

* Add "experimental" features to docs.rs

docs.rs/bindgen doesn't contain the documentation for things under the `"experimental"` feature. This means that no-one can see the `emit_diagnostics` replacement for `emit_warnings`.

* Update Cargo.toml

Maybe we just need `all-features = true`?

Author: CGMossa

bindgen/Cargo.toml

@@ -59,3 +59,6 @@ __cli = []
 __testing_only_extra_assertions = []
 __testing_only_libclang_9 = []
 __testing_only_libclang_5 = []
+
+[package.metadata.docs.rs]
+features = ["experimental"]