[docs] check: add webservice and admin setting docs

[matthewhilton]

Related to changes in https://tracker.moodle.org/browse/MDL-67898

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[andrewnicols]

I strongly recommed wrapping this prematurely early, for example:

Suggested change

	$settings->add(new admin_setting_check('myplugin/usefulcheck', (new \mod\myplugin\check\usefulcheck())->get_ref()));
	$settings->add(
	new admin_setting_check(
	'myplugin/usefulcheck',
	(new \mod\myplugin\check\usefulcheck())->get_ref(),
	),
	);

The reason to do this so early is that we don't know what size window someone will be reading docs in and it's nicer to not have examples wrap because they're in a scrollable div. If things are too wide for that window, it's hard to read the example clearly.

[sarjona]

Hi @matthewhilton!
I tried to update the patch with Andrew's proposal but then I realised there are some conflicts with the changes that were integrated in #939 a few weeks ago.

It would be great if you could rebase your patch or close it (whatever works better for you) ;-)
Thanks a lot for your help! :-)