Basic: docstring: not all objects in SymPy are Basic

MutableDenseMatrix and MutableSparseMatrix are not for example.
> Is this a bug or a feature? Unless we can decide on this, I'm -1.

Feature, I gather.  But my citation is from 2007: https://github.com/sympy/sympy/issues/3519#issuecomment-36984041, so if someone knows otherwise...  See also my notes in #8283.