What does "developed in discernment" mean?
The word translated "developed in discernment" is bhāvitapañño, i.e. increased or developed pañña.

The dictionary entry for pañña says,


  of wisdom, endowed with knowledge or insight, possessed of the highest cognition


The dictionary definition lists a lot of places where it's used in various compounds, (possibly describing various types of wisdom or insight) -- including the bhāvita compound, which is used in for example SN 35.127 (in summary, immune to sexual desire, and guarding the senses) and AN 10.24 (in summary, immune to various unwholesome mental factors).

I'd say these describe the effect of wisdom rather than describing the object of wisdom -- but, that's not surprising because what it's describing is "developed wisdom" i.e. the effect of having perfected wisdom.

I guess you'd have to look at other places pañña is used by itself (to find a description of pañña itself), and/or other pañña compounds (for descriptions of the various types of pañña).1

Wikipedia says it means "insight into the three characteristics": quoting Gombrich and others.

It's one of the several perfections (i.e. virtues which can be "perfected" or completed).



1 Not easy to summarise:


A search of SuttaCentral returns 500 results -- many from the Abhidamma and so on.
AN 8.2 describes 8 different types of discernment (which may confirm my theory that there are many different types or aspects of wisdom, that's it's not a term with a narrow meaning)
Pañña is an entire branch of the Threefold Training, so I expect it includes right view and right intention