Re-add documentation for JSDoc and $props()

[theetrain]

Describe the problem

The documentation addition made in #11370 (for issue #10541) seems to have disappeared.

Describe the proposed solution

I hope we can bring this back, including:

• How to document props via JS or TS
• How to document components via @component

Importance

nice to have

[dummdidumm]

The first one is available here: https://svelte.dev/docs/svelte/$props#Type-safety
Second one is gone indeed and should be added back.