vlc_stream: fix the vlc_stream_ReadPartial() documentation

vlc_stream_ReadPartial() is similar to pf_read of stream_t. It is often used as such in pf_read callbacks.

vlc_stream_Read() uses it that way as well. On negative return values (-1) it continues reading.

Internally it's calling vlc_stream_ReadRaw() which also returns pf_read return values verbatim.

extracted from !2110 (closed)