[PATCH] documentation for clocktable :tags syntax

[Antti Kaihola <akaihola@gmail.com>]

After some trial and error I had to dig the mailing lists to figure
out the syntax of the :tags clock table option. I propose an addition
to the documentation along these lines:

diff --git a/doc/org.texi b/doc/org.texi
index 7142ce0..bec1123 100644
--- a/doc/org.texi
+++ b/doc/org.texi
@@ -5892,7 +5892,8 @@ be selected:
              @r{To use this, @code{:block} or @code{:tstart},
@code{:tend} are needed.}
 :stepskip0   @r{Do not show steps that have zero time.}
 :fileskip0   @r{Do not show table sections from files which did not
contribute.}
-:tags        @r{A tags match to select entries that should contribute}.
+:tags        @r{A tags match to select entries that should contribute.}
+             @r{See @ref{Matching tags and properties} for the match syntax.}
 @end example

 Then there are options which determine the formatting of the table.  There