Partially Fixes Camel 14995: generates master and 3/threescale traits docs.

[djencks]

This adjusts the source locations in script/gen_doc.sh to actually find the master and threescale source and thus generate the docs. The solution seems to be to list each source directory explicitly. I haven't found any documentation on --input-dirs or been able to discover where it is used or what format it is supposed to be in.

This is seriously incomplete for a robust solution:

• possible other traits in addons need to be added to gen_doc.sh by hand. This IMO is not likely to happen.
• There is no check that every checked in .adoc that was once generated is still generated. In fact I have found no evidence that gen_docs.sh is run as part of ci.