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.
* 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.

1 files changed, 5 insertions, 4 deletions

diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index 88bde6e8ce4..0cc4a55dc99 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -2908,10 +2908,6 @@ impl<T: Clone> From<&mut [T]> for Vec<T> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "vec_from_array", since = "1.44.0")]
 impl<T, const N: usize> From<[T; N]> for Vec<T> {
-    #[cfg(not(test))]
-    fn from(s: [T; N]) -> Vec<T> {
-        <[T]>::into_vec(box s)
-    }
     /// Allocate a `Vec<T>` and move `s`'s items into it.
     ///
     /// # Examples
@@ -2919,6 +2915,11 @@ impl<T, const N: usize> From<[T; N]> for Vec<T> {
     /// ```
     /// assert_eq!(Vec::from([1, 2, 3]), vec![1, 2, 3]);
     /// ```
+    #[cfg(not(test))]
+    fn from(s: [T; N]) -> Vec<T> {
+        <[T]>::into_vec(box s)
+    }
+
     #[cfg(test)]
     fn from(s: [T; N]) -> Vec<T> {
         crate::slice::into_vec(box s)