Rollup merge of #89869 - kpreid:from-doc, r=yaahc

Add documentation to more `From::from` implementations.

For users looking at documentation through IDE popups, this gives them relevant information rather than the generic trait documentation wording “Performs the conversion”. For users reading the documentation for a specific type for any reason, this informs them when the conversion may allocate or copy significant memory versus when it is always a move or cheap copy.

Notes on specific cases:
* The new documentation for `From<T> for T` explains that it is not a conversion at all.
* Also documented `impl<T, U> Into<U> for T where U: From<T>`, the other central blanket implementation of conversion.
* The new documentation for construction of maps and sets from arrays of keys mentions the handling of duplicates. Future work could be to do this for *all* code paths that convert an iterable to a map or set.
* I did not add documentation to conversions of a specific error type to a more general error type.
* I did not add documentation to unstable code.

This change was prepared by searching for the text "From<... for" and so may have missed some cases that for whatever reason did not match. I also looked for `Into` impls but did not find any worth documenting by the above criteria.

library/core/src/task/poll.rs

@@ -243,7 +243,7 @@ impl<T, E> Poll<Option<Result<T, E>>> {
 #[stable(feature = "futures_api", since = "1.36.0")]
 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 impl<T> const From<T> for Poll<T> {
-    /// Convert to a `Ready` variant.
+    /// Moves the value into a [`Poll::Ready`] to make a `Poll<T>`.
     ///
     /// # Example
     ///