HTMLMediaElement waitingforkey event docs

[hamishwillee]

This adds docs for the HTMLMediaElement event waitingforkey, and tidied some grammar for the encrypted event.

Related to docs work for FF133 in #36558

[github-actions[bot]]

Preview URLs

• /en-US/docs/Web/API/HTMLMediaElement/encrypted_event
• /en-US/docs/Web/API/HTMLMediaElement/waitingforkey_event
• /en-US/docs/Web/API/HTMLMediaElement

Flaws (8)

Note! *2 documents with no flaws that don't need to be listed. 🎉* URL: [`/en-US/docs/Web/API/HTMLMediaElement`](https://pr36838.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/HTMLMediaElement) Title: `HTMLMediaElement` Flaw count: 8 - **macros**: - `/en-US/docs/Web/API/HTMLMediaElement/played does not exist` - `/en-US/docs/Web/API/HTMLMediaElement/seeking does not exist` - `/en-US/docs/Web/API/MediaController does not exist` - `/en-US/docs/Web/API/MediaController does not exist` - `/en-US/docs/Web/API/HTMLMediaElement/mozAudioCaptured does not exist` - *and 3 more flaws omitted*