Add clarification text to options

[chipbennett]

Per this WPORG support forum topic:
http://wordpress.org/support/topic/a-notice-missing?replies=1#post-4505834

I think there should be a notice like

these option will work ONLY if the options "E-mail me whenever" above are checked

just below these checkboxes:

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Because if I am not wrong, it works this way.