`struct` documentation should include `Sized` in the listed auto trait implementations

[eggyal]

This was previously raised in #17606, which was closed when auto-traits were documented in #47833—however, since Sized is not a library-implemented auto-trait but rather a compiler-implemented trait, it was omitted from that solution.

The issue appears to have also been previously raised in #41537 (closed as duplicate of the above) and #75413 (I think erroneously closed as duplicate of #24183).

@rustbot label A-docs C-bug

[eggyal]

@cafce25 has pointed out to me (on StackOverflow) that !Sized is listed for unsized types.

[veera-sivarajan]

@rustbot label -needs-triage +T-docs-rs