pcomplete: add custom completions?

[Kai Grossjohann]

I'm using pcomplete for shell-mode.  I have custom shell commands that
pcomplete doesn't know about.  How do I add them to pcomplete?  (A
pointer where to start reading would be enough.)

Background: with shell-integration.el some shell commands are actually
intercepted and executed in Lisp.  This is the case for all functions
named shell/foo -- the command foo will run the function shell/foo.

Kai