improved documentation of the cobs decoding

[gutzchi]

I believe that the modification of the input buffer by the cobs decoding functions is not expected as a user of the library, as it requires you to copy the input buffer every time you call it. In postcard, this does not happen with the non-cobs serde functions.

I consider that by noting this fact in the documentation, other users might be able to realise it earlier and spend less time debugging as it happened to me.

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[jamesmunns]

This looks like a very reasonable addition, thanks!