fix: correct the comment for resizeToFit

[wy-luke]

According to the actual effect of resizeToFit(), I think it would be better to change the comment to to fit it's content rather to fit container.

The TextBlock will always be resized to fit it's content whether the text is too long or too large. Any excess beyond the container, like a plane attached the texture with one TextBlock, will not be displayed.

In the meanwhile, when I set the fontSize, width and resizeToFit simultaneously, it's width to fit fontSize, rather fontSize to fit width.

let text = new BABYLON.GUI.TextBlock();
text.text = "Hello world";
text.fontSize = 50;
text.width = 0.5
text.resizeToFit = true

It's all about the content. There's nothing about the container or to fit container.

As far as I'm concerned, the original version is kind of ambiguous to understand. And I think it might be better to make this modification for better clarity.

Thanks.

[wy-luke]

In Unity, there is a Best Fit option for text control. It makes the fontSize to fit the control size. In other words, you don't need to set the font size, it will be determined by the textBlock control size.

This is also one of the reason why I misunderstood the resizeToFit method in the beginning. And then, this comment made me even more confused.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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