cobs.h: add documentation for cobs_encode() and cobs_decode()

[Lotterleben]

Add some documentation to clarify which information ends up where after en/decoding. I tried to match your comment style (going full-on Doxygen seemed a little over the top for a minimal library like this one), if you prefer a different format I'd be happy to change my PR acoordingly :)
I also added some line breaks to the function signature to match the line length hinted at by your /***************************************************************************** comment blocks.

[cmcqueen]

Thanks for the PR. That's a good improvement.

The same improvement could be made to the functions themselves in cobs.c. Also the corresponding COBS/R functions in cobsr.h and cobsr.c. Do you want to make that change to the PR? Otherwise I'm happy to take this PR as-is, then I would make the update to the other files myself.

[Lotterleben]

The same improvement could be made to the functions themselves in cobs.c.

Do you mean adding the same text to the .c files? Sure, no problem.

Also the corresponding COBS/R functions in cobsr.h and cobsr.c.

Good point, that makes a lot of sense.

Do you want to make that change to the PR? Otherwise I'm happy to take this PR as-is, then I would make the update to the other files myself.

I'll be happy to make the changes you requested.

[Lotterleben]

Oh! I've had a look at your issues just now, maybe this PR resolves #3? :)

[cmcqueen]

Thanks very much for your contribution!

For issue #3, I intended a wiki page with further description and examples of usage.

[Lotterleben]

Oh right :)
Thanks for merging! I could've squashed all commits into one, sorry if I messed up your commit history.