Document MicroPython `@asm_pio` syntax explicitly

[Wren6991]

Originally raised on Discord. There is generally a 1:1 relationship between the syntax inside @asm_pio decorators in MicroPython, and the instructions listed in the datasheet, but there are some wrinkles like the use of in_() for in instructions due to this being a reserved keyword in Python.

I think we need to expand the MP PIO docs to call out the syntax explicitly (now that it is stable) and generally provide more non-code-example docs of the PIO API.

[lurch]

Is this now sufficiently covered by https://docs.micropython.org/en/latest/rp2/tutorial/pio.html ?