[plt-scheme] note: change in doc.txt handling for Help Desk

In the past, Help Desk searched in all collections (and no where else)
for doc.txt files. In preparation for better PLaneT integration, Help
Desk now looks in info.ss files for doc.txt to be defined and uses that
definition to find doc.txt files. The change is released to CVS and
documented, but just in case you make doc.txt files for your own
collections, be sure to add an info.ss file directing Help Desk to find
your doc.txt file.

Robby