Clarify rationale: what software is affected by 'com.google.fonts/check/varfont/has_HVAR' ?

[felipesanches]

Rationale currently states that "Not having a HVAR table can lead to costly text-layout operations on some platforms, which we want to avoid."

I would like to improve that text mentioning which specific platforms (and perhaps which versions of them) are actually affected by this.

I would be glad if you could hel us clarify that, @nyshadhr9, as you were the original proponent for that new check.

Cheers, Felipe