CXX-988 Fix documentation of element::operator[]
CXX-988 Fix documentation of element::operator[]

It does not throw for invalid element access.

mongodb/mongo-cxx-driver,xdg/mongo-cxx-driver,mongodb/mongo-cxx-driver,acmorrow/mongo-cxx-driver,acmorrow/mongo-cxx-driver,acmorrow/mongo-cxx-driver,acmorrow/mongo-cxx-driver,xdg/mongo-cxx-driver,xdg/mongo-cxx-driver,mongodb/mongo-cxx-driver,mongodb/mongo-cxx-driver,xdg/mongo-cxx-driver