feat(kbkdf): enable missing_docs lint

[TheBestTvarynka]

Hi,
I enabled missing_docs lint for the kbkdf crate and fixed compiler warnings.

closes #129

[newpavlov]

We don't need the rust_2018_idioms lint, since IIUC the lints are already enabled by default on the edition which we use.

[TheBestTvarynka]

We don't need the rust_2018_idioms lint, since IIUC the lints are already enabled by default on the edition which we use.

@newpavlov I already fixed it (force pushed). My fork was not updated, and I missed that it is enabled

[newpavlov]

Thank you!