[DOC] Mathtext and matshow examples

I did this because of #2024.
When rebasing, I saw the mathtext example has already been improved in 40340fc, so no problem if the consensus is to close this without merging (since I had completed the work I thought I would do the PR anyway).


The "Writing mathematical expressions" tutorial is of high quality, so I felt it would be better to try to improve the related example in the screenshot http://matplotlib.org/users/screenshots.html.
I have checked pep8 and that the pdf output is correct when running `python mathtext_examples.py --latex` (amsmath and amssymb packages are needed).

I attach the result of doc building, since it is sometimes tricky to figure it out: ![mathtext_examples_01_00](https://f.cloud.github.com/assets/2669128/565237/b01817d6-c5f4-11e2-8b1c-087b6f69f4e6.png)

The matshow example with its 2 oversized figures suffers from a similar problem, so I took this opportunity to modify it as well.