plugin/log: detail {combined} and {common} (#2422)

These can be used (and are handy), document them.
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/log/README.md

@@ -27,7 +27,9 @@ log [NAME] [FORMAT]
 ~~~
 
 * `NAME` is the name to match in order to be logged
-* `FORMAT` is the log format to use (default is Common Log Format)
+* `FORMAT` is the log format to use (default is Common Log Format), `{common}` is used as a shortcut
+  for the Common Log Format. You can also use `{combined}` for a format that adds the query opcode
+  `{>opcode}` to the Common Log Format.
 
 You can further specify the classes of responses that get logged:
 
@@ -45,7 +47,8 @@ The classes of responses have the following meaning:
 * `denial`: either NXDOMAIN or NODATA (name exists, type does not)
 * `error`: SERVFAIL, NOTIMP, REFUSED, etc. Anything that indicates the remote server is not willing to
     resolve the request.
-* `all`: the default - nothing is specified. Using of this class means that all messages will be logged whatever we mix together with "all".
+* `all`: the default - nothing is specified. Using of this class means that all messages will be
+  logged whatever we mix together with "all".
 
 If no class is specified, it defaults to *all*.
 
@@ -75,8 +78,8 @@ The following place holders are supported:
 * `{>opcode}`: query OPCODE
 * `{/[LABEL]}`: any metadata label is accepted as a place holder if it is enclosed between `{/` and  `}`.
 the place holder will be replaced by the corresponding metadata value or the default value `-` if label is not defined.
-
-
+* `{common}`: the default Common Log Format.
+* `{combined}`: the Common Log Format with the query opcode.
 
 The default Common Log Format is:
 
@@ -119,11 +122,11 @@ Only log denials for example.org (and below to a file)
 }
 ~~~
 
-Log all queries which were not resolved successfully
+Log all queries which were not resolved successfully in the Combined Log Format.
 
 ~~~ corefile
 . {
-    log . {
+    log . {combined} {
         class denial error
     }
 }