Clarify documentation for sentinel behavior of `*encode*` fns

[starthal]

The documentation describes that encode, try_encode, etc. "use" the default sentinel value (or override it). IMO they should clarify that the functions do not actually write the sentinel to the buffer.

This is documented in CobsEncoder::finalize but not the free functions that use it.

[rockboynton]

I agree, I would really expect encode to append the sentinel as is the point of COBS