Add usage example/doctest to ByteBuffer docstring

This only passes as a doctest in Python 3 because of the bytestring
literals for some of the expected values. In Python 2, those values are
simply regular strings and doctest seems to work by comparing the string
representations of the values rather than the values themselves, so
those lines fail because they don't have the `b` prefix.

smart_open/bytebuffer.py

@@ -17,7 +17,31 @@ class ByteBuffer(object):
 
     The bytes are stored in a bytestring, and previously-read bytes are freed
     when the buffer is next filled (by slicing the bytestring into a smaller
-    copy)."""
+    copy).
+
+
+    Example:
+    >>> buf = ByteBuffer(chunk_size = 8)
+    >>> message_bytes = iter([b'Hello, W', b'orld!'])
+    >>> buf.fill(message_bytes)
+    8
+    >>> len(buf) # only chunk_size bytes are filled
+    8
+    >>> buf.peek()
+    b'Hello, W'
+    >>> len(buf) # peek() does not change read position
+    8
+    >>> buf.read(6)
+    b'Hello,'
+    >>> len(buf) # read() does change read position
+    2
+    >>> buf.fill(message_bytes)
+    5
+    >>> buf.read()
+    b' World!'
+    >>> len(buf)
+    0
+    """
 
     def __init__(self, chunk_size=io.DEFAULT_BUFFER_SIZE):
         """Create a ByteBuffer instance that reads chunk_size bytes when filled.