Adds HTMLCanvasElement as possible value for pointStyle option in the documentation

[stockiNail]

Fixes #9457

[LeeLenaleee]

Maby instead of specifying the htmlcanvaselement it might be better to place a reference to all the kind attributes the drawImage method accepts since it also accepts videoElements and offscreenCanvasElements for example

[stockiNail]

@LeeLenaleee good point! But please forgive my poor knowledge of javascript.

In helper.canvas.js there is a check about the object type which is passed:

Chart.js/src/helpers/helpers.canvas.js

Lines 132 to 142 in b32fb48

	if (style && typeof style === 'object') {
	type = style.toString();
	if (type === '[object HTMLImageElement]' || type === '[object HTMLCanvasElement]') {
	ctx.save();
	ctx.translate(x, y);
	ctx.rotate(rad);
	ctx.drawImage(style, -style.width / 2, -style.height / 2, style.width, style.height);
	ctx.restore();
	return;
	}
	}

and I think only HTMLCanvasElement and HTMLImageElement are allowed.

Am I wrong?

[LeeLenaleee]

No seems like it's my bad, should have looked up code first, somehow I just thought it would get passed along and just accept anything the drawImage accepts