altera_avalon_pio_regs.h functions

I don't know if "official" information from Altera exists about all of functions included in HAL drivers. Usually you can find the basic function to access IP cores if you browse .h files.

You can also find a lot of information if you search the forum: I usually find all I need in older posts. If the search is ineffective you can possibly start a new thread and often you get the right answer in short time.

The core user guides are useful if you need to access the module at the hw level, I mean directly reading and writing hw register, thus bypassing HAL driver. Sometimes, knowledge of the underlying hardware is also useful to pull out maximum performance from the core.