SpiDevice: add guidelines for behaviour of CS pin upon bus failures

[newAM]

Currently the SpiDevice trait does not define the sequence of actions that occur after a bus error.

I discussed this with @Dirbaio in the matrix chat and the conclusion we reached was that this should be added to the documentation as a guideline to match what already exists for ExclusiveDevice. The rationale for a guideline instead of a requirement is that SPI error recovery is implementation specific.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @eldruin (or someone else) soon.

Please see the contribution instructions for more information.

[Dirbaio]

LGTM, could you do the same change to async's docs ?

[newAM]

LGTM, could you do the same change to async's docs ?

Good call. Made the change & rebased.

[newAM]

Should I add a changelog entry for this? I wasn't sure because it doesn't change anyone's impl.

[Dirbaio]

IMO it's small enough that no changelog is fine, but feel free to add it if you want.

[eldruin]

Looks good to me, thank you!
I agree no changelog entry is necessary especially since we are still in alpha phase.
bors r+

[bors]

Build succeeded:

• ci-linux (1.54.0, x86_64-unknown-linux-gnu)
• ci-linux (stable, thumbv6m-none-eabi)
• ci-linux (stable, thumbv7m-none-eabi)
• ci-linux (stable, x86_64-unknown-linux-gnu)
• ci-linux-test (1.54.0, x86_64-unknown-linux-gnu)
• ci-linux-test (stable)
• fmt