step-9: Improve the documentation and typography.

step-9: Improve the documentation and typography.

1. Remove an inaccurate remark on SUPG: The gradient of test functions scales
   like 1/h, so scaling it by h makes the term O(1), not O(h).
2. General improvements: Reword some sentences and fix some typos.
3. Remark on the choice of diffusion coefficient.
4. Notate floating point numbers as, e.g., '0.'.
5. Clarify text on TensorFunction. AFAIK this class never returned anything but
   Tensors so this documentation was incorrect.
6. Use face_n instead of face or face_no to indicate the face number.
7. Move the Tensor projected_gradient closer to where it is actually used and
   remove a remark about allocating it.
8. Mark an unchanged Tensor (gradient) as const.
9. Prefer p.norm() to std::sqrt(p.norm_square()).
10. Add 'using namespace dealii;' to improve the doxygen result. Since we strip
   all preceding dealii namespace qualifiers this prevents doxygen from writing

       ::MultithreadInfo::set_thread_limit();

   which looks odd.
11. Clarify that Assert aborts (it does not terminate).