[css3-flexbox] 'align-self' property definition should be clearer 'auto' margins override it

http://dev.w3.org/csswg/css3-flexbox/#align-self doesn't mention
anywhere that auto margins in the cross axis override 'align-self'.

However, the algorithm, in particular
http://dev.w3.org/csswg/css3-flexbox/#cross-sizing steps 8 and 11,
and http://dev.w3.org/csswg/css3-flexbox/#cross-alignment steps 13
and 14, make it clear that align-self has no effect if either
cross-axis margin is 'auto'.

I think this should be mentioned in the description of the property
so that more casual readers will be aware of it.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                           http://www.mozilla.org/   𝄂