Documentation on reneging stream active mark may need update.

[huitema]

Reneging happens when an application marks a stream active, but cannot send data when receiving the "prepare to send" callback. A frequent case is when the proposed buffer is too small, and the application wants to wait for a larger buffer. The documentation in "send_receive_data.md" says that the application can do one of two things:

• either call picoquic_provide_stream_data_buffer while requesting 0 bytes buffer and setting the `still_active' flag,

    /* Not sending here! */
    (void)picoquic_provide_stream_data_buffer(context, 0, 0, 1);

• or call picoquic_mark_active_stream from within the call back, which according to the documentation is equivalent.

Except it does not actually seem equivalent, according to
this discussion. Trying the "mark active" approach appears to cause the application to stall.

We need to either update the documentation or fix the code to ensure that the "mark active" approach also works.

[huitema]

Addressed in PR #1822