platformio.project.helpers does not seem to be documented

[Pharap]

I recently raised platformio/platformio-core#2971 where it was recommended that I use platformio.project.helpers ("PIO Core API"), but this feature doesn't seem to be documented anywhere on docs.platformio.org.

[ivankravets]

Thanks for the issue! A temporary solution is to look over functions here https://github.com/platformio/platformio-core/blob/develop/platformio/project/helpers.py