Clarify cubeb_data_callback ret value for input only streams.
Clarify cubeb_data_callback ret value for input only streams.

The description of the ret value of cubeb_data_callback specifies that it is
the number of frames written to the output buffer. However, in the input only
stream case no data should be written to the output buffer and the output
buffer arg will be NULL.

This leaves it unclear as to the value that should be returned in the input
only case. To prevent a drain nframes should be returned, however, reading the
documentation left me unsure as to if I should return 0 or nframes.

This commit attempts to clarify the doc comment for input only streams.

kinetiknz/cubeb,kinetiknz/cubeb,kinetiknz/cubeb