Here's a heuristic that works for me, for $foobar, any language and/or sufficiently complicated tech:

Within the documentation, find the sections that deal with the following topics:

  * extending $foobar
  * embedding C in $foobar
  * embedding $foobar in C
  * packaging/distributing $foobar
  * porting/cross-building $foobar

In my experience, these topics are inherently hardest to implement, last to be documented, and trickiest to understand. As such, they make for worthwhile reading for advanced practitioners.