Clarify course->enrol meaning

course->enrol option in course settings page accept "yes/no/date range".

The help file simply says: "Whether this course is enrollable or not. With a third option of allowing enrolment within a range of date".

While playing with enrol-plugins I discovered that this setting only has meaning for "internal-auto-enrol" option. Else, if using any type of external enrollment, this setting, simply, is ignored.

Something about this behavior should be written to the corresponding help file (courseenrollable.html).

Ciao