add documentation

[jimy-byerley]

Hello everyone, thanks for having wrapped the etherlab kernel module !

I'm starting this PR to add rust documentation to the crate. My first motivation is to have at least some description so I can easier use that crate in a project of mine, so my documentation efforts will be driven by my specific need. The description I will write might eventually be oriented for my use when my understanding is not that good, so I will try to avoid this.

For now I only documented Master, Domain, SlaveConfig by copying the descriptions from the functions of etherlab's C library

[birkenfeld]

Thanks for the PR!

The issue I see here is that by copying from the GPL-licensed Etherlab implementation verbatim, we would have to relicense this crate or be in violation of the GPL.

Since I'm not a lawyer, I'm not sure how much the wording would need to differ to avoid this issue; this is the main reason #8 is still open.

[jimy-byerley]

I'm not a lawyer either, but it seems we are in quite a dumb situation, there is not thousands of clear and concise ways of saying the same things. Does the license really apply on the the comments in the code ? Maybe I can tune those comments to have some different sentences than the original, but a dumb work it is :sweat: