docs: document public apis for cyw43 driver

cyw43-pio/README.md

@@ -0,0 +1,17 @@
+# cyw43-pio
+
+RP2040 PIO driver for the nonstandard half-duplex SPI used in the Pico W. The PIO driver offloads SPI communication with the WiFi chip and improves throughput.
+
+## Minimum supported Rust version (MSRV)
+
+Embassy is guaranteed to compile on the latest stable Rust version at the time of release. It might compile with older versions but that may change in any new patch release.
+
+## License
+
+This work is licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  <http://www.apache.org/licenses/LICENSE-2.0>)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>)
+
+at your option.

cyw43-pio/src/lib.rs

@@ -1,5 +1,7 @@
 #![no_std]
 #![allow(async_fn_in_trait)]
+#![doc = include_str!("../README.md")]
+#![warn(missing_docs)]
 
 use core::slice;
 
@@ -11,6 +13,7 @@ use embassy_rp::{Peripheral, PeripheralRef};
 use fixed::FixedU32;
 use pio_proc::pio_asm;
 
+/// SPI comms driven by PIO.
 pub struct PioSpi<'d, CS: Pin, PIO: Instance, const SM: usize, DMA> {
     cs: Output<'d, CS>,
     sm: StateMachine<'d, PIO, SM>,
@@ -25,6 +28,7 @@ where
     CS: Pin,
     PIO: Instance,
 {
+    /// Create a new instance of PioSpi.
     pub fn new<DIO, CLK>(
         common: &mut Common<'d, PIO>,
         mut sm: StateMachine<'d, PIO, SM>,
@@ -143,6 +147,7 @@ where
         }
     }
 
+    /// Write data to peripheral and return status.
     pub async fn write(&mut self, write: &[u32]) -> u32 {
         self.sm.set_enable(false);
         let write_bits = write.len() * 32 - 1;
@@ -170,6 +175,7 @@ where
         status
     }
 
+    /// Send command and read response into buffer.
     pub async fn cmd_read(&mut self, cmd: u32, read: &mut [u32]) -> u32 {
         self.sm.set_enable(false);
         let write_bits = 31;