[docs] Improve documentation around capability contextlevel in the docs

What is the URL of the page?

https://moodledev.io/docs/5.0/apis/subsystems/access#how-to-define-new-capabilities-in-plugins

What is the issue with this page?

In the dev chat there was a discussion around what contextlevel on capabilities does and means we seemed to come up with a good description that is not included in the docs at the moment:

It indicates what context you intend to check the capability at, and does control where it appears, for instance moodle/site:config is defined as CONTEXT_SYSTEM and that means you can't change it at lower levels. If you go to a course (or category, or module or block, or user for that matter) and try to override capabilities there, that one doesn't show on the list. Because if you did override it in a course, it wouldn't make any difference when you check it as context system.

It seems that adding this or something similar will help people understand what contextlevel they should choose to use better.

Are you able to provide a patch for this?

If no one picks this up sooner, I'll likely do it next week

Open the linked capability documentation page and start at “How to define new capabilities in plugins,” reviewing the existing contextlevel guidance. Add a clear explanation of what contextlevel controls, including why system-level capabilities cannot be overridden lower in the context hierarchy, then verify the rendered documentation.