In quickstart of the documentation it says:

The Blender/Python API can’t (yet)…

• Define callbacks or listeners to be notified when data is changed.

However, callbacks on data changes are possible according to the Properties documentation:
https://docs.blender.org/api/blender2.8/bpy.props.html#update-example

Are these 2 pages talking about a different aspect or is it a conflicting statement? I went looking for ways to trigger functions on data change, but started to look outside Blender for ways to do that due to quickstart doc. However, by chance, I stumbled on the other page to find out it IS possible in Blender.