moodle-tool_mergeusers - #37: moving button help from form to page header

Related to issue #37:

* A better place for the button help, considering John contributions,
I think it is better to place it in the header instead of the form.
Merge pull request #38 from jpahullo/issue37

moodle-tool_mergeusers - #37: moving button help from form to page header