DOC: add a couple plt.close('all') to avoid warning
Some small doc fixes:
- added some `plt.close('all')` calls to avoid the matplotlib warning that there are more than 20 figures open.
- some style errors in release.rst. Apparantly sphinx complains about something like ```NaN``s`.because there may not be a character directly after the closing ````

@y-p OK to merge? (before I have to rebase release.rst with this pace of development)
 Look fine, go ahead.
