Document %I and %O ncsa format specifiers

doc/sphinx/reference/varnishncsa.rst

@@ -57,6 +57,9 @@ Supported formatters are:
 %h
   Remote host. Defaults to '-' if not known.
 
+%I
+  Total bytes received from client.
+
 %{X}i
   The contents of request header X.
 
@@ -72,6 +75,9 @@ Supported formatters are:
 %{X}o
   The contents of response header X.
 
+%O
+  Total bytes sent to client.
+
 %r
   The first line of the request. Synthesized from other fields, so it
   may not be the request verbatim.