DOC: add a couple plt.close('all') to avoid warning

[jorisvandenbossche]

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 ````

[jorisvandenbossche]

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

[ghost]

Look fine, go ahead.