Add docstrings on box classes.
Add docstrings on box classes.

prepare/TestWeasyPrint,andrewleech/WeasyPrint,andrewleech/WeasyPrint,Kozea/WeasyPrint,prepare/TestWeasyPrint,jasco/WeasyPrint,marclaporte/WeasyPrint,marclaporte/WeasyPrint,Kozea/WeasyPrint,jasco/WeasyPrint