plugin/log: update nodata/NXDOMAIN story a bit (#2416)

* plugin/log: update nodata/NXDOMAIN story a bit

Add more words on nodata and NXDOMAIN which is a confusing DNS
"feature"

Fixes: #2412
Signed-off-by: Miek Gieben <miek@miek.nl>

* typos
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/log/README.md

@@ -44,7 +44,8 @@ log [NAME] [FORMAT] {
 The classes of responses have the following meaning:
 
 * `success`: successful response
-* `denial`: either NXDOMAIN or NODATA (name exists, type does not)
+* `denial`: either NXDOMAIN or nodata responses (Name exists, type does not). A nodata response
+   sets the return code to NOERROR.
 * `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
@@ -76,10 +77,11 @@ The following place holders are supported:
 * `{>do}`: is the EDNS0 DO (DNSSEC OK) bit set in the query
 * `{>id}`: query ID
 * `{>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.
+* `{/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. See the *metadata* plugin for more information.
 
 The default Common Log Format is:
 
@@ -112,7 +114,7 @@ Custom log format, for all zones (`.`)
 }
 ~~~
 
-Only log denials for example.org (and below to a file)
+Only log denials (NXDOMAIN and nodata) for example.org (and below)
 
 ~~~ corefile
 . {