improve rationale for 'com.google.fonts/check/has_MVAR'

[felipesanches]

"Per the spec, the MVAR tables contain variation data for metadata otherwise in like OS/2 and hhea; if not present, then the default values in those tables will apply to all instances, which can effect text layout."

@davelab6, this description is not very clear to be used as a rationale for the check. Do you have a clearer way to express this idea? I will create the check using this text, but I would like to later further improve it.