The customizing.txt file is getting large and difficult to find
information in it.

It is a mix of "how to do something": Change cache-control headers, add 
timelog class,  adding external authentication, spam prevention ...

and manual on schema methods and classes, tracker configuration, tal 
methods, detectors, access controls ....

I added indexing to it but there is still a long ways to go. I think
this document is trying to do too much. Maybe split it into a
reference manual (extended design doc...) and recipes for doing
things would be helpful?

A lot of 'doing things' examples exists on the wiki as well and
it would be good to reference interesting items from there in the
docs.