bpo-33864: Clarify the docs for typing.ByteString (GH-21311)

(cherry picked from commit b40e434386cd94a367d4a256e3364771140160e7)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>
diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst
index e85b6d6..e5143c5 100644
--- a/Doc/library/typing.rst
+++ b/Doc/library/typing.rst
@@ -672,7 +672,7 @@
    A generic version of :class:`collections.abc.ByteString`.
 
    This type represents the types :class:`bytes`, :class:`bytearray`,
-   and :class:`memoryview`.
+   and :class:`memoryview` of byte sequences.
 
    As a shorthand for this type, :class:`bytes` can be used to
    annotate arguments of any of the types mentioned above.