Document that BytesMut::reserve may not preserve memory outside 0..len

Author: paolobarbolini

src/bytes_mut.rs

@@ -551,6 +551,9 @@ impl BytesMut {
     /// and the original buffer is large enough to fit the requested additional
     /// capacity, then reallocations will never happen.
     ///
+    /// Once this method has been called, the contents of memory outside of
+    /// `0..len` are undefined and may be uninitialized.
+    ///
     /// # Examples
     ///
     /// In the following example, a new buffer is allocated.