"erl -man io" doesn't work?

While I was playing around with erlang, I found my installation of erlang's man page does not work:

$ erl -man io
No manual entry for io

I have compiled and installed (with configure's prefix option) in /opt/erlang on Solaris 9, put the man page files just in /opt/erlang/man.  Clearly this doesn't work.  So I did some tracking with:

$ truss -o /tmp/output.txt erl -man io 

(truss is like strace for Solaris; "strace" on Solaris is completely different thing.)

And found that the "/man" directory should live in /opt/erlang/lib/erlang/man.

"erl -man io" works now.  It's in MANPATH and Windex is created -- so just "man io" would work just fine, but I just wanted to fix it anyway.