Add remarks about callbacks and spdylay_session_{recv,send}

doc/apiref-header.rst

@@ -7,3 +7,13 @@ Includes
 To use the public APIs, include ``spdylay/spdylay.h``::
 
     #include <spdylay/spdylay.h>
+
+Remarks
+-------
+
+Do not call `spdylay_session_send`, `spdylay_session_recv` or
+`spdylay_session_mem_recv` from the spdylay callback functions
+directly or indirectly. It will lead to the crash. You can submit
+requests or frames in the callbacks then call `spdylay_session_send`,
+`spdylay_session_recv` or `spdylay_session_mem_recv` outside of the
+callbacks.