Webservice documentation improvement

I run an issue with how to properly parametrize the call to "core_enrol_submit_user_enrolment_form" webservice.

It should be good if all the webservices would have a detailed documentation with samples.

For example the "core_enrol_submit_user_enrolment_form" webservice has not enough data fro use.

The description is: Submit form data for enrolment form
REST (POST parameters):
formdata= string

Fact, the manual enrolment form uses the following data:  id (at table: user_enrolments), enrolid, userid, rolid, timestart, timeend,
but I did not find any workable combination of the parameterize "formdata"

I used the call:
/webservice/rest/server.php?moodlewsrestformat=json&wstoken={token}&wsfunction=core_enrol_submit_user_enrolment_form&formdata=%7B%22ue%22:%22740%22,%22timestart%22:%22%22,%22timeend%22:%221661990399%22%7D
The last part of the call-url is (in PHP): 
'&formdata={"ue":"'.$enrolments_id.'","timestart":"'.$unixtimestart.'","timeend":"'.$unixtimeend.'"}';

But it may wrong because nowhere documented the formdata what really is?

I am receiving the following error:
            [exception] => dml_missing_record_exception
            [errorcode] => invalidrecord
            [message] => The user_enrolments table does not contain records.
            [debuginfo] => SELECT * FROM {user_enrolments} WHERE id IS NULL

And, it looks like the  SELECT function did not work in 'user_enrolments' table without passing the id value.
How to pass id value properly?

So, it is the reason why I suggested ti improve webservice documentaiton.