Update Chain parameter tab help text

Automation chains can have parameters. This can be useful when you want to write re-usable chains. Declare some parameters to your chain, reference the values of those parameters using the syntax "
@

" and then execute your chain from another one using the operations of the category "Execution Flow", that have a "parameters" attribute where you can set the value of the parameters, like the "Run Chain" operation. See a step by step example
http://doc.nuxeo.com/x/wgEuAQ