[Bug]: BUILDING.md does not explain the test targets, APP_ROOT, or the effects of running the integration tests

[michaelzhang114]

I have done the following

• I have searched the existing issues
• If possible, I've reproduced the issue using the 'main' branch of this project

Steps to reproduce

BUILDING.md

Problem description

BUILDING.md's "Compile and test" section gives a single command, make APP_ROOT=test-data all test integration, without explaining:

• what make all, make test, and make integration each do, or that make integration
  runs bin/container and therefore needs make all first
• what APP_ROOT is for, that make integration erases its contents before each run, and
  that leaving it unset makes the integration tests operate on the user's real application
  data directory

Environment

- OS: 26.6.2
- Xcode: 27.0
- Container: 1.2.0

Code of Conduct

• I agree to follow this project's Code of Conduct