Rollup merge of #53518 - phungleson:fix-impl-from-for-convert, r=frewsxcv

Add doc for impl From in char_convert

As part of issue #51430 (cc @skade).

The impl is very simple, let me know if we need to go into any details.

Author: pietroalbini

src/libcore/char/convert.rs

@@ -115,6 +115,19 @@ pub unsafe fn from_u32_unchecked(i: u32) -> char {
 
 #[stable(feature = "char_convert", since = "1.13.0")]
 impl From<char> for u32 {
+    /// Converts a [`char`] into a [`u32`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::mem;
+    ///
+    /// fn main() {
+    ///     let c = 'c';
+    ///     let u = u32::from(c);
+    ///     assert!(4 == mem::size_of_val(&u))
+    /// }
+    /// ```
     #[inline]
     fn from(c: char) -> Self {
         c as u32
@@ -141,6 +154,19 @@ impl From<char> for u32 {
 /// C0 and C1 control codes.
 #[stable(feature = "char_convert", since = "1.13.0")]
 impl From<u8> for char {
+    /// Converts a [`u8`] into a [`char`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::mem;
+    ///
+    /// fn main() {
+    ///     let u = 32 as u8;
+    ///     let c = char::from(u);
+    ///     assert!(4 == mem::size_of_val(&c))
+    /// }
+    /// ```
     #[inline]
     fn from(i: u8) -> Self {
         i as char