rust: pin-init: fix broken rust doc link

Rust 1.92.0 warns when building the documentation that [`PinnedDrop`] is
an invalid reference. This is correct and it's weird that it didn't warn
before, so fix the link.

[ The reason is that it is hidden -- I had asked about that in the
upstream PR that changed the behavior because I wasn't sure it was
intentional (and thus whether we needed to fix this and other cases):

https://github.com/rust-lang/rust/pull/147153#issuecomment-3395484636

It turns out it was not, and it has been fixed for 1.92.0's upcoming
release thanks to Guillaume and León. So we do not strictly need
this patch and the other changes anymore:

https://github.com/rust-lang/rust/pull/147809

However, checking hidden/private items or, even better, a runtime
toggle to be able to see those on the fly, is something that I think
would be quite nice so I have had it in our usual lists for a while.
Guillaume is open to the idea and perhaps experimenting with an
implementation on our side first -- he asked me to open issues
upstream:

https://github.com/rust-lang/rust/issues/149105
https://github.com/rust-lang/rust/issues/149106

- Miguel ]

Signed-off-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/20251016211740.653599-1-lossin@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>