Revert "Remove note about Stream/Serial deprecation"

This reverts commit 5ee5b07.

Author: Jenny Plunkett

drivers/Serial.h

@@ -82,6 +82,10 @@ class Serial : public SerialBase, public Stream, private NonCopyable<Serial> {
      */
     Serial(PinName tx, PinName rx, int baud);
 
+    /* Stream gives us a FileHandle with non-functional poll()/readable()/writable. Pass through
+     * the calls from the SerialBase instead for backwards compatibility. This problem is
+     * part of why Stream and Serial should be deprecated.
+     */
     bool readable()
     {
         return SerialBase::readable();