This function seems to work fine in multi-core applications now.
The delay is now in units of microseconds instead of depending on the clock
speed, and is adjustable by board configuration headers.
Also added documentation.
All the method signatures from rp2_pio.c and friends have been taken and
converted to RST format, then explanatory notes added for each signature.
Signed-off-by: Tim Radvan <tim@tjvr.org>