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/lazy.rs

@@ -75,6 +75,7 @@ impl<T: Eq> Eq for OnceCell<T> {}
 
 #[unstable(feature = "once_cell", issue = "74465")]
 impl<T> const From<T> for OnceCell<T> {
+    /// Creates a new `OnceCell<T>` which already contains the given `value`.
     fn from(value: T) -> Self {
         OnceCell { inner: UnsafeCell::new(Some(value)) }
     }