Elaborate on bytes-like objects.

Doc/glossary.rst

@@ -79,8 +79,12 @@ Glossary
       <http://www.python.org/~guido/>`_, Python's creator.
 
    bytes-like object
-      An object that supports the :ref:`bufferobjects`, like :class:`bytes` or
-      :class:`bytearray`.
+      An object that supports the :ref:`bufferobjects`, like :class:`bytes`,
+      :class:`bytearray` or :class:`memoryview`.  Bytes-like objects can
+      be used for various operations that expect binary data, such as
+      compression, saving to a binary file or sending over a socket.
+      Some operations need the binary data to be mutable, in which case
+      not all bytes-like objects can apply.
 
    bytecode
       Python source code is compiled into bytecode, the internal representation