Highlight API should explicitly prohibit layout changes

[SergeyMalkin]

There are issues with text shaping broken across element boundaries, see this for example. Also, behavior in this case varies significantly between browser or browser versions:

Highlight API should explicitly prohibit changes in layout when highlight is added, it must only have effect on rendering.

[BoCupp-Microsoft]

Thanks for calling out this scenario. I'll update the document.

[BoCupp-Microsoft]

@SergeyMalkin do you want to review this PR and let me know if it addresses this issue? Feel free to suggest better language if you don't feel I've covered your concerns completely. Thanks!

[gked]

Closing as Sergey's feedback has been addressed.