ping_callback is a member of stream (API Change):

fix #446

* The ping_callback option struct is removed.

Actions Required:

* Change call sites which use ping_callback with set_option to
  call stream::ping_callback instead.

doc/quickref.xml

@@ -124,7 +124,6 @@
           <bridgehead renderas="sect3">Options</bridgehead>
           <simplelist type="vert" columns="1">
             <member><link linkend="beast.ref.websocket__permessage_deflate">permessage_deflate</link></member>
-            <member><link linkend="beast.ref.websocket__ping_callback">ping_callback</link></member>
           </simplelist>
           <bridgehead renderas="sect3">Constants</bridgehead>
           <simplelist type="vert" columns="1">