Improved documentation/doctests in tensor/modules

- Finish to add doctests in underscored methods in tensor/modules

- Move import of sage/tensor/modules/all.py from sage/all.py to
  sage/tensor/all.py

- Move import of sage/geometry/manifolds/all.py from sage/all.py
  to sage/geometry/all.py

- Remove the (spurious) mentions "This file was autogenerated" in rst files