Improve description of region capture API

[foolip]

These are changes suggested by @eladalon1983 after I asked about the current description, which I thought seemed wrong in parts.

[ddbeck]

I wrote the original description. It's not great, but I don't think it's wrong. CropTarget is the unavoidable entry point, as far as I can tell.

Here are two new variations for your consideration. The first tries to illustrate the relationship between CropTarget and the video track:

Suggested change

	description: |
	Region Capture is an API for cropping video tracks derived from screen-capturing a tab.
	The API consists of two parts.
	First, a `CropTarget` object is minted from an HTMLElement `e`, representing an element within the captured tab.
	Second, `cropTo()` is invoked on the video track, with the aforementioned `CropTarget` as input, after which frames deliver on that track will be cropped to `e`'s bounding box.
	Calling `cropTo(null)` restores the track to its original, uncropped form.
	description: The `CropTarget.fromElement()` static method selects an element to be cropped from a display-capture video track by the track's `cropTo()` method. For example, use it to crop out video conferencing controls during screen sharing.

This one throws up its hands and says, it's complicated:

Suggested change

	description: |
	Region Capture is an API for cropping video tracks derived from screen-capturing a tab.
	The API consists of two parts.
	First, a `CropTarget` object is minted from an HTMLElement `e`, representing an element within the captured tab.
	Second, `cropTo()` is invoked on the video track, with the aforementioned `CropTarget` as input, after which frames deliver on that track will be cropped to `e`'s bounding box.
	Calling `cropTo(null)` restores the track to its original, uncropped form.
	description: The region capture API crops an element, such as an element containing video conferencing controls, from a captured video track.

[eladalon1983]

The proposal is shorter and has some benefits, but I worry that it could be misunderstood, especially around the word "select". Calling CropTarget.fromElement() itself has no effect; one could call it multiple times with different inputs and see no change. It's only when the returned token is fed into cropTo() that an effect follows. If the suggestion could be modified to better reflect this, I think that would be best.

[eladalon1983]

"crops an element ... from"

With this suggestion, I worry that the reader could get the impression that the element is removed from the capture, whereas it's actually everything else that gets removed.