Reword docs for PropertyWithMixin

This should emphasize that the ignore list refers to the mixins, not
the properties (as in the case of the `inline-block` mixin, which is
only recommended when writing `display: inline-block;`).

Change-Id: I2f52882cb07d0bea404f47f437bec20bef93f931
Reviewed-on: http://gerrit.causes.com/40959
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>