Remove SPI3 from variants. Clarify module guidelines

README.md

@@ -87,17 +87,19 @@ PRs are encouraged. Documenting each step using reference manuals is encouraged,
 Most peripheral modules use the following format:
 
 - Enums for various config settings, that implement `#[repr(u8)]` for their associated register values
-- A peripheral struct that owns the reg block, and has fields for config. This struct includes
+- A peripheral struct that has public fields for config. This struct also includes
 a private `regs` field that is the appropriate reg block. Where possible, this is defined generically
 in the implementation, eg:
-`U: Deref<Target = pac::usart1::RegisterBlock>,`. [Reference the stm32-rs-nightlies Githug](https://github.com/stm32-rs/stm32-rs-nightlies)
+`U: Deref<Target = pac::usart1::RegisterBlock>,`. Reference the [stm32-rs-nightlies Github](https://github.com/stm32-rs/stm32-rs-nightlies)
 to identify when we can take advantage of this.
-- If config fields are complicated, we use a separate Config struct owned by the peripheral struct.
-This Config struct impls `Default`.
+- If config fields are complicated, we use a separate `PeriphConfig` struct owned by the peripheral struct.
+This struct impls `Default`.
 - A constructor named `new` that performs setup code, including RCC peripheral enable and reset
-- `enable_interrupt` and `clear_interrupt` functions
-- `embedded-hal` implementations as required, that call native methods as applicable. Note that
-we design our APIs based on STM32 capabilities, and apply EH traits as applicable.
+- `enable_interrupt` and `clear_interrupt` functions, which accept an enum of interrupt type
+- `embedded-hal` implementations as required, that call native methods. Note that
+we design APIs based on STM32 capabilities, and apply EH traits as applicable.
+- When available, base setup and usage steps on instructions provided in Reference Manuals.
+These steps are copy+pasted in comments before the code that performs each one.
 
 
 ## Errata

src/spi.rs

@@ -36,6 +36,7 @@ pub enum Error {
 pub enum SpiDevice {
     One,
     Two,
+    #[cfg(not(any(feature = "f3x4", feature = "g0")))]
     Three,
 }
 
@@ -146,18 +147,17 @@ where
         clocks: &C,
         rcc: &mut RCC,
     ) -> Self {
-        // l4x3 and L5 support SPI3, but have an inconsitent naming convention for enabling rcc.
-        // (ie `sp3en` instead of `spi3en`.)
         match device {
             SpiDevice::One => {
                 rcc_en_reset!(apb2, spi1, rcc);
             }
             SpiDevice::Two => {
                 rcc_en_reset!(apb1, spi2, rcc);
             }
+            #[cfg(not(any(feature = "f3x4", feature = "g0")))]
             SpiDevice::Three => {
                 cfg_if! {
-                    // Note the difference of `sp3en` mixed with `spi3rst`.
+                    // Note `sp3en` mixed with `spi3rst`; why we can't use the usual macro.
                     if #[cfg(any(feature = "l4x3", feature = "l5"))] {
                         rcc.apb1enr1.modify(|_, w| w.sp3en().set_bit());
                         rcc.apb1rstr1.modify(|_, w| w.spi3rst().set_bit());