Hi all, I want to filter my refile targets to "Tasks" headings.  This
snippet works:
https://lists.gnu.org/r/emacs-orgmode/2016-02/msg00088.html

There's a few things I'd like to improve on:

1) My org-agenda-files show up in the list.  For example, foo.org and
bar.org show up in the refile targets, despite the function should return
nil if a heading does not contain "Tasks"

2) I would like to limit the "Task" headings to those that have a parent
heading with a tag :project:
I have :project: tag as exclude-from-inheritance, so this might be tricky

3) I would like the parent heading with :project: tag to have a TODO state
that is not "done" and is not "cancelled".

Can anyone help me improve on this?  I think I'm missing some really easy
documentation / examples.  As an aside, is there a definitive "Org Mode
API" doc that shows the functions that would assist me in writing a
function to perform the above filtering?  Something like a Javadoc for
Org-Mode?  There's great articles, etc. but I think it would be cool to
have a "public" API defined / documented - otherwise it seems that I'm
simply going through existing blogs, documentation etc and of course the
source code.  When going through source code, there's many functions that
are named similarly and yes there's docs but I found myself wanting some
kind of Javadoc-ish API documentation :)

Thanks,
--Nate