List of required parameters for web service requests

I am having trouble finding a list the required parameters for each api call.

For example, this call works fine, as it does not require any parameters:

core_course_get_courses

However, as another example, when I attempt any of these three, I get an error because I do not know what parameters I need to submit with it:

core_enrol_get_enrolled_users

core_enrol_get_users_courses

core_enrol_search_users

These three calls return:

[exception] => invalid_parameter_exception

[errorcode] => invalidparameter

[message] => Invalid parameter value detected

So where can I find these api calls with a list of the required parameters for each.