improve documentation for `std::pin::Pin`

[vincenzopalazzo]

Summary

As @yoshuawuyts pointed out on Zulip stream the in struct, there is a lack of documentation on some of the methods such as Pin::new() and Pin::into_inner

This should be relatively straight forward to document, and I can mentor someone if there is interest

[vincenzopalazzo]

@rustbot label +E-mentor

[ch-iv]

I can try doing this

[ch-iv]

@rustbot claim

[vincenzopalazzo]

Fixed in #105902