Update doc

_sources/apiref.txt

@@ -722,7 +722,8 @@ Types (structs, unions and typedefs)
     without blocking, it must return :macro:`NGHTTP2_ERR_WOULDBLOCK`. If
     it gets EOF before it reads any single byte, it must return
     :macro:`NGHTTP2_ERR_EOF`. For other errors, it must return
-    :macro:`NGHTTP2_ERR_CALLBACK_FAILURE`.
+    :macro:`NGHTTP2_ERR_CALLBACK_FAILURE`. Returning 0 is treated as
+    :macro:`NGHTTP2_ERR_WOULDBLOCK`.
 
 .. type:: typedef int (*nghttp2_on_frame_recv_callback) (nghttp2_session *session, const nghttp2_frame *frame, void *user_data)
 

apiref.html

@@ -1113,7 +1113,8 @@ written in <em>buf</em> if it succeeds. If it cannot read any single byte
 without blocking, it must return <a class="reference internal" href="#NGHTTP2_ERR_WOULDBLOCK" title="NGHTTP2_ERR_WOULDBLOCK"><tt class="xref c c-macro docutils literal"><span class="pre">NGHTTP2_ERR_WOULDBLOCK</span></tt></a>. If
 it gets EOF before it reads any single byte, it must return
 <a class="reference internal" href="#NGHTTP2_ERR_EOF" title="NGHTTP2_ERR_EOF"><tt class="xref c c-macro docutils literal"><span class="pre">NGHTTP2_ERR_EOF</span></tt></a>. For other errors, it must return
-<a class="reference internal" href="#NGHTTP2_ERR_CALLBACK_FAILURE" title="NGHTTP2_ERR_CALLBACK_FAILURE"><tt class="xref c c-macro docutils literal"><span class="pre">NGHTTP2_ERR_CALLBACK_FAILURE</span></tt></a>.</p>
+<a class="reference internal" href="#NGHTTP2_ERR_CALLBACK_FAILURE" title="NGHTTP2_ERR_CALLBACK_FAILURE"><tt class="xref c c-macro docutils literal"><span class="pre">NGHTTP2_ERR_CALLBACK_FAILURE</span></tt></a>. Returning 0 is treated as
+<a class="reference internal" href="#NGHTTP2_ERR_WOULDBLOCK" title="NGHTTP2_ERR_WOULDBLOCK"><tt class="xref c c-macro docutils literal"><span class="pre">NGHTTP2_ERR_WOULDBLOCK</span></tt></a>.</p>
 </dd></dl>
 
 <dl class="type">

nghttp2.h.html

@@ -860,7 +860,8 @@
 <span class="cm"> * without blocking, it must return :enum:`NGHTTP2_ERR_WOULDBLOCK`. If</span>
 <span class="cm"> * it gets EOF before it reads any single byte, it must return</span>
 <span class="cm"> * :enum:`NGHTTP2_ERR_EOF`. For other errors, it must return</span>
-<span class="cm"> * :enum:`NGHTTP2_ERR_CALLBACK_FAILURE`.</span>
+<span class="cm"> * :enum:`NGHTTP2_ERR_CALLBACK_FAILURE`. Returning 0 is treated as</span>
+<span class="cm"> * :enum:`NGHTTP2_ERR_WOULDBLOCK`.</span>
 <span class="cm"> */</span>
 <span class="k">typedef</span> <span class="nf">ssize_t</span> <span class="p">(</span><span class="o">*</span><span class="n">nghttp2_recv_callback</span><span class="p">)</span>
 <span class="p">(</span><span class="n">nghttp2_session</span> <span class="o">*</span><span class="n">session</span><span class="p">,</span>