Comment clarification in test class

Comment clarification in test class

I just updated the comments on this helper to clarify the limitations of the function & the
fact that it should not be our only way to test thousand separators.

I was noticing perfect was becoming the enemy of the good here. The function was marked as deprecated
because it doesn't cover all scenarios - but the upshot was that we stopped increasing out
thousand separator testing. In fact we need lots of form tests to do some testing of
the separators and a very small number to test more variants - this latter has been added
& the comments point to the need for more without going as far as deprecating