texdoctk local texmf

Dear List,

I'm struggling trying to configure texdoctk to recognize the documentation in 
my local texmf tree (/usr/local/share/texmf/doc). I've peeked at the texdocrc 
file, which has these relevant lines:

# root of doc directory (from $TEXMFMAIN)
TEXDOCPATH=../doc/texmf
# root of local doc directory (from $TEXMFLOCAL)
# can be left empty if equal to TEXDOCPATH or if $TEXMFLOCAL is undefined
LOCALDOCPATH=

Issuing the command "kpsewhich -expand-var='$TEXMF'" confirms that my local 
tree is in the search path, so I supposed leaving LOCALDOCPATH empty should 
be OK. It's not, so I made a copy of this file as ~/.texdocrc and explicitly 
set that variable to my local documentation tree (see above). It's still not 
OK.

The man page for texdoctk points to a texdoc-local.dat file that can be set 
up, but no instructions on how to do that. I'd like to keep my main texmf 
tree intact, so I can smoothly upgrade tetex in the future. Any help setting 
this up would be greatly appreciated.

Cheers,
Sebastian