Add override details to validate's docstring
Add override details to validate's docstring

edx-solutions/XBlock,nagyistoce/edx-XBlock,4eek/XBlock,EDUlib/XBlock,edx/XBlock,4eek/XBlock,mitodl/XBlock,nagyistoce/edx-XBlock,EDUlib/XBlock,lovehhf/XBlock,mitodl/XBlock,lovehhf/XBlock,edx/XBlock,edx-solutions/XBlock