Make the `Mv.make_x` functions private
Make the `Mv.make_x` functions private

These are not documented or used anywhere outside of __init__.
Nor are they useful, since the caller can just construct themselves a new `Mv` instead of reusing the old one.

Now that they are private, we can convert these comments into docstrings without them appearing on readthedocs.

arsenovic/galgebra,arsenovic/galgebra