Document workaround for Text class that avoids tinted background color

[ReignOfError]

The backgroundColor of a Text instance's textField (if set using Text.setTextProperty) will be tinted using Text.color. A workaround to avoid this behaviour is to set (and use) Text.richText (this prevents Text from calling the updateColorTransform function inherited from the Image class).

It would be useful for this behaviour to be documented, perhaps under the description for Text.setTextProperty since anyone wishing to change the background color of text will presumably consult the documentation for this method at some point.

For more details, see http://developers.useflashpunk.net/t/text-color-possible-bug-textcolor-property-set-via-text-settextproperty-is-ignored/550