Add a more precise definition of audible element

[youennf]

As mentioned by @alastor0325 in https://github.com/w3c/audio-session/pull/23#discussion_r1805525746, it would be nice to:

• Have a more detailed description of what audible means
• Add links from the definition of audio session elements to the section detailing how HTMLMediaElement/AudioContext/... are integrated.