doc: Document expected return value of channel data callback
doc: Document expected return value of channel data callback

Signed-off-by: Audrius Butkevicius <f90e40bb91d386c08b3311f23196fd604c51ee0f@gmail.com>
Reviewed-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

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