Extended the docs for mapping attributes precision and scale

I for myself am always confused about precision and scale and have to lookup what is the numer of maximum digits and what is the number for digits right of the decimal point. Other people seem to have the same [problem](http://stackoverflow.com/questions/14940574/what-do-scale-and-precision-mean-when-specifying-a-decimal-field-type-in-doctrin).

So I have extended the docs to make it more clear.