I take chapter "19.12 base64—RFC 3548: Base16, Base32, Base64 Data
Encodings" (Python 2.6) as an example. At the bottom of the chapter, you
find references to other resouces, in this case:

See Also:
Module binascii Support module containing ASCII-to-binary and
binary-to-ASCII conversions.
RFC 1521 - MIME (Multipurpose Internet Mail Extensions) Part One:
Mechanisms for Specifying and Describing the Format of Section 5.2,
“Base64 Content-Transfer-Encoding,” provides the definition of the
base64 encoding.

Interestingly enough RFC 1521 points to a web page (the link has been
removed in the current HTML documentation).

However "Module binascii" does not link to binascii in the library.pdf,
whereas the HTML documentation nicely links to
http://docs.python.org/library/binascii.html.

The file base64.rst IMO indicates that a link should be produced:
.. seealso::

   Module :mod:`binascii`
      Support module containing ASCII-to-binary and binary-to-ASCII
conversions.

   :rfc:`1521` - MIME (Multipurpose Internet Mail Extensions) Part One:
Mechanisms for Specifying and Describing the Format of Internet Message
Bodies
      Section 5.2, "Base64 Content-Transfer-Encoding," provides the
definition of the
      base64 encoding.