where are the docs for $HG environment variable?

[Forest]

I just upgraded my windows box to mercurial 0.9.4 from a recent 0.9.3
snapshot, and found that hgk was broken.  It seems that, despite already
having the hg executable in my path, I now also have to set the HG
environment variable for this to work.  

(This was not obvious from the error message, by the way.  I figured it out
mainly because I happened to have been browsing the mercurial-devel archives
last night, and read a thread about set_hgexecutable on windows.)

Where is $HG documented?  I don't see it in the latest man pages, and I'd
like to know the rules for its use.  Which commands require it?  Can I use
an hgrc entry instead of setting an environment variable?

I updated the HgkExtension wiki page in hopes of saving other windows users
the frustration of figuring this out:
http://www.selenic.com/mercurial/wiki/index.cgi/HgkExtension?action=diff