Contribution description

This PR aims to add a little bit of documentation about the gpio_get_pin_num and gpio_get_port, which are not really documented but incredibly important if you want to use a function like gpio_ll_init.

The issue first came up during the development of #21332, because the nRF52 microcontrollers don't actually distinguish between port P0 and P1 internally, there are just 32 registers for the pins and P1 has the fifth bit set in the pin number (Mask 0x1F).
Therefore, if you try to initialize a P1 pin with gpio_ll_init and just naively use numbers for the pins, you will actually modify the pins on P0.
It just came up in our seminar again, therefore I decided to finally add some documentation.

Also I fixed the documentation for gpio_irq_enable, which was accidentally rendered as a code block instead of text. See: https://www.doxygen.nl/manual/markdown.html#md_codeblock

Testing procedure

Make sure the documentation still looks good.

Issues/PRs references

None.