Missing interface documentation for push/pull functions

[Florian Weimer]

The documentation should mention that the functions must return -1
(and not any other negative value, especially not GNUTLS error codes)
on error, and set errno to a suitable error code.  The calling
functions do not handle other negative return values very well.

-- 
Florian Weimer                <fweimer at bfk.de>
BFK edv-consulting GmbH       http://www.bfk.de/
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