g.parser: doc of rules (merge: https://trac.osgeo.org/grass/changeset/63180)

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@63183 15284696-431f-4ddb-bdfa-cd5b030d7da7

general/g.parser/g.parser.html

@@ -98,6 +98,17 @@ can be easily defined as
 #%end
 </pre></div>
 
+The parser allows to define predefined <em>rules</em>
+for used options.
+
+The syntax of the rules section is following:
+
+<div class="code"><pre>
+#%rules
+#% exclusive: capfile_output, capfile
+#%end
+</pre></div>
+
 <h2>NOTES</h2>
 
 An option can be instructed to allow multiple inputs by adding the