Add doctest example for Buffer::from_bytes (apache#6920)

* Add doctest example for

* Remove typo

* Update arrow-buffer/src/buffer/immutable.rs

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

arrow-buffer/src/buffer/immutable.rs

@@ -60,6 +60,14 @@ unsafe impl Sync for Buffer where Bytes: Sync {}
 
 impl Buffer {
     /// Auxiliary method to create a new Buffer
+    ///
+    /// This can be used with a [`bytes::Bytes`] via `into()`:
+    ///
+    /// ```
+    /// # use arrow_buffer::Buffer;
+    /// let bytes = bytes::Bytes::from_static(b"foo");
+    /// let buffer = Buffer::from_bytes(bytes.into());
+    /// ```
     #[inline]
     pub fn from_bytes(bytes: Bytes) -> Self {
         let length = bytes.len();