Clarify that `ManuallyDrop<T>` has the same bit validity as `T`.

The `unfold` function have since been renamed to `from_fn`.

…tions, r=workingjubilee

`waitqueue` clarifications for SGX platform

The documentation of `waitqueue` functions on the `x86_64-fortanix-unknown-sgx` platform is incorrect at some places and on others missing. This PR improves upon this.

cc: `@jethrogb`

Clarify example in docs of str::char_slice

Just a one word improvement.

“Last” can be misread as meaning the last (third) instead of the previous (first).

Clarify ManuallyDrop bit validity

Clarify that `ManuallyDrop<T>` has the same bit validity as `T`.

…ttmcm

Fix a comment in std::iter::successors

The `unfold` function have since rust-lang#58062 been renamed to `from_fn`.
(I'm not sure if this whole comment is still useful—it's not like there are many iterators that *can't* be based on `from_fn`. Anyway, in its current form this comment is not correct, and it sent me into a half-hour research of what happened to `unfold` function, so I want to do *something* with it 🙃 deleting these three lines is a perfectly fine alternative, in my opinion.)

update some comments around swap()

Based on `@eddyb's` comment [here](rust-lang/unsafe-code-guidelines#461 (comment)).

And then I noticed the wrong capitalization for Miri and fixed it in some other places as well.

…l, r=GuillaumeGomez

rustdoc-search: fix bug with multi-item impl trait

Preview searches:

- https://notriddle.com/rustdoc-html-demo-5/compiler-doc-impl-trait-bugfix/index.html?search=-%3E%20globalctxt

- https://notriddle.com/rustdoc-html-demo-5/compiler-doc-impl-trait-bugfix/index.html?search=globalctxt