Update license field information in documentation

[keunes]

Replace the explicit list of accepted SPDX identifiers for the 'license' field with a link to the add-on server documentation.

Some of the values currently listed will return errors when submitted, e.g.:

WebExtError: Submission failed (2): Bad Request
{
  "version": {
    "license": [
      "License with slug=GPL-3.0-or-later does not exist."
    ]
  }
}

[willdurand]

@rebloor thoughts?

[rebloor]

I would be more specific about the link so the reader knows they're in the right place when they get there

Suggested change

	See the [add-on server documentation](https://mozilla.github.io/addons-server/topics/api/licenses.html#license-list) for which values are accepted in the `"license"` field.
	The values accepted by the `"license"` field are listed in the [License Choices - non-Themes](https://mozilla.github.io/addons-server/topics/api/licenses.html#license-list) section of the addons-server API documentation.