clarify spacing around mfrac

Created by: fred-wang

Currently, the implementation note gives 0 spacing around fractions unless the numerator is an embellished operator (in that case the lspace/rspace values are used).

WebKit uses 0 spacing and at the moment embellished operator spacing is not currently implemented.

Gecko always sets a minimum of 1px spacing around fractions (and adds the embellished op spacing to this 1px if necessary). This allows for example to distinguish \frac{a}{b}\frac{c}{d} VS \frac{ac}{bd}.

At the moment this uncertainty makes difficult to write frac tests that work on all browsers.

@khaledhosny: what do you think about it? Should we add this minimum spacing?