Add content for "Plain Body Data" input vector

Signed-off-by: kingthorin <kingthorin@users.noreply.github.com>

Author: kingthorin

addOns/help/src/main/javahelp/contents/ui/dialogs/options/ascaninput.html

@@ -27,6 +27,9 @@ <h5>Add URL Query Parameter?</h5>
 <h4>POST Data</h4>
 Key value pairs in the request POST data.
 
+<h4>Plain Body Data</h4>
+When selected ZAP will process requests with body content and either no or text/plain content-type as a single input/parameter for testing purposes.
+
 <h4>URL Path</h4>
 Path elements in the request URL, ie the elements separated by <code>/</code>.