Update doc

doc/apiref-header.rst

@@ -14,8 +14,18 @@ The header files are also available online: :doc:`nghttp2.h` and
 Remarks
 -------
 
-Do not call `nghttp2_session_send`, `nghttp2_session_mem_send`,
-`nghttp2_session_recv` or `nghttp2_session_mem_recv` from the nghttp2
-callback functions directly or indirectly. It will lead to the
+Do not call `nghttp2_session_send()`, `nghttp2_session_mem_send()`,
+`nghttp2_session_recv()` or `nghttp2_session_mem_recv()` from the
+nghttp2 callback functions directly or indirectly. It will lead to the
 crash.  You can submit requests or frames in the callbacks then call
 these functions outside the callbacks.
+
+Currently, `nghttp2_session_send()` and `nghttp2_session_mem_send()`
+do not send client connection preface
+(:macro:`NGHTTP2_CLIENT_CONNECTION_PREFACE`).  The applications are
+responsible to send it before sending any HTTP/2 frames using these
+functions if :type:`nghttp2_session` is configured as client.
+Similarly, `nghttp2_session_recv()` and `nghttp2_session_mem_recv()`
+do not consume client connection preface.  The applications are
+responsible to receive it before calling these functions if
+:type:`nghttp2_session` is configured as server.