Add documentation to Box conversions

Among other changes, documents whether allocations are necessary
to complete the type conversion.

Part of rust-lang#51430

Co-authored-by: Giacomo Stevanato <giaco.stevanato@gmail.com>

Co-authored-by: Joshua Nelson <github@jyn.dev>

library/alloc/src/boxed.rs

@@ -1277,6 +1277,7 @@ impl<T> From<T> for Box<T> {
     /// from the stack into it.
     ///
     /// # Examples
+    ///
     /// ```rust
     /// let x = 5;
     /// let boxed = Box::new(5);
@@ -1330,6 +1331,12 @@ impl<T: Copy> From<&[T]> for Box<[T]> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_from_cow", since = "1.45.0")]
 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
+    /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
+    ///
+    /// When `cow` is the `Cow::Borrowed` variant, this
+    /// conversion allocates on the heap and performs a copy of the
+    /// underlying slice. Otherwise, it will try to re-use the owned
+    /// vec's allocation.
     #[inline]
     fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
         match cow {
@@ -1348,6 +1355,7 @@ impl From<&str> for Box<str> {
     /// and performs a copy of `s`.
     ///
     /// # Examples
+    ///
     /// ```rust
     /// let boxed: Box<str> = Box::from("hello");
     /// println!("{}", boxed);
@@ -1361,6 +1369,29 @@ impl From<&str> for Box<str> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_from_cow", since = "1.45.0")]
 impl From<Cow<'_, str>> for Box<str> {
+    /// Converts a `Cow<'_, str>` into a `Box<str>`
+    ///
+    /// When `cow` is the `Cow::Borrowed` variant, this
+    /// conversion allocates on the heap and performs a copy of the
+    /// underlying `str`. Otherwise, it will try to re-use the owned
+    /// `String`'s allocation.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use std::borrow::Cow;
+    ///
+    /// let unboxed = Cow::Borrowed("hello");
+    /// let boxed: Box<str> = Box::from(unboxed);
+    /// println!("{}", boxed);
+    /// ```
+    ///
+    /// ```rust
+    /// # use std::borrow::Cow;
+    /// let unboxed = Cow::Owned("hello".to_string());
+    /// let boxed: Box<str> = Box::from(unboxed);
+    /// println!("{}", boxed);
+    /// ```
     #[inline]
     fn from(cow: Cow<'_, str>) -> Box<str> {
         match cow {
@@ -1403,6 +1434,7 @@ impl<T, const N: usize> From<[T; N]> for Box<[T]> {
     /// This conversion moves the array to newly heap-allocated memory.
     ///
     /// # Examples
+    ///
     /// ```rust
     /// let boxed: Box<[u8]> = Box::from([4, 2]);
     /// println!("{:?}", boxed);
@@ -1416,6 +1448,14 @@ impl<T, const N: usize> From<[T; N]> for Box<[T]> {
 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
     type Error = Box<[T]>;
 
+    /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
+    ///
+    /// The conversion occurs in-place and does not require a
+    /// new memory allocation.
+    ///
+    /// # Errors
+    ///
+    /// Returns the old `Box<[T]>` in the `Err` variant if `boxed_slice.len()` does not equal `N`.
     fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
         if boxed_slice.len() == N {
             Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })