io: clarify ReadBuf::uninit allows initialized buffers as well (#7053)

tokio/src/io/read_buf.rs

@@ -39,9 +39,11 @@ impl<'a> ReadBuf<'a> {
         }
     }
 
-    /// Creates a new `ReadBuf` from a fully uninitialized buffer.
+    /// Creates a new `ReadBuf` from a buffer that may be uninitialized.
     ///
-    /// Use `assume_init` if part of the buffer is known to be already initialized.
+    /// The internal cursor will mark the entire buffer as uninitialized. If
+    /// the buffer is known to be partially initialized, then use `assume_init`
+    /// to move the internal cursor.
     #[inline]
     pub fn uninit(buf: &'a mut [MaybeUninit<u8>]) -> ReadBuf<'a> {
         ReadBuf {