cmov: add correct usage examples for both `Cmov` and `CmovEq`

I plan to get these done later today also, and they'll follow the tests as a rough example (but obviously cleaner, easier to understand and commented so users know what's going on).

I plan to add warnings about using Cmov directly, as it would act more of an optimisation barrier if a condition is set to x == y.

I may also add warnings about branching in general, and using the ^ operator where the functions provided by CmovEq aren't adequate (which will be extremely unlikely but still probably worth noting). I'll also briefly explain why using == is bad, but I'm no branching expert so that may require some tweaking.

I'll try to adhere to the RustCrypto example format in order to reduce PR review time, and for consistency.

I think I'll try and get #920 fixed up first though.

Start by reading the existing tests for Cmov and CmovEq, then compare their structure with other RustCrypto examples. Add clear, commented usage examples for both APIs and document the cautions around direct Cmov use, == conditions, branching, and the ^ operator; the examples should follow the repository's established format.