There's a note that BEP 11 is the pending draft for PEX. However this was never published, and there's no official specification that I can find.
There is no official specification, but other clients have documented the de-facto standard of the ut_pex-over-ltep extension.
http://svn.tribler.org/abc/branches/boudewijn/beps/bep_0034.rst
https://trac.transmissionbt.com/browser/trunk/doc/extended-messaging.txt?rev=7196
Update: Now there also is an official spec.