Auto merge of #117138 - zachs18:rwlock_guard_debug_unsized, r=dtolnay

Add T: ?Sized to `RwLockReadGuard` and `RwLockWriteGuard`'s Debug impls.

For context, `MutexGuard` has `+ ?Sized` on its `Debug` impl, and all three have `+ ?Sized` on their `Display` impls.

It looks like the `?Sized` was just missed when the impls were added (the impl for `MutexGuard` was added in the same PR (rust-lang/rust#38006) with support for `T: Debug + ?Sized`, and `RwLock*Guard`s did allow `T: ?Sized` types already); the `Display` impls were added later (rust-lang/rust#42822) with support for `T: Debug + ?Sized` types.

I think this needs a T-libs-api FCP? I'm not sure if this also needs an ACP. If so I can make one.

These are changes to (stable) trait impls on stable types so will be insta-stable.

`@rustbot` label +T-libs-api

Author: bors