DOC:Use monospace for --

The -- is rendering as an en dash on http://matplotlib.org/users/pyplot_tutorial.html

Maybe all strings should use monospace instead of single quotes?
DOC: Change all strings to ``'monospace'`` in quotes
DOC: Change strings to monospace
Adding possibility to remove invisible lines and patches from relim
DOC: Add axes_api to documentation after the refactoring

Recently, axes was converted from a module to a package.
This adds axes_api to the doctree back.
`show-inheritance` was removed because it tells the user that
`Axes` `Bases: matplotlib.axes._base._AxesBase`
which is not that relevant I think.
This is mentioned in issue #1461
Adding test test_relim_visible_only
DOC: changed axes_api from automodule to autoclass

with automodule we had: `matplotlib.axes._axes.Axes`
while with autoclass we have `matplotlib.axes.Axes`

However, we must remember this documents only one class, so if
classes will be added to the module, they should be added
by hand.
Merge pull request #2242 from endolith/patch-3

DOC:Use monospace for --
Merge pull request #2417 from fariza/remlim-with-invisible-option

Adding possibility to remove invisible lines and patches from relim.
Merge pull request #2427 from jarondl/add-Axes-api-doc

DOC: Add axes_api to documentation after the refactoring
Merge remote-tracking branch 'upstream/v1.3.x'
Use six.string_types instead of basestring.
Merge pull request #2439 from alefnula/remove-basestring

Use six.string_types instead of basestring.
Merge remote-tracking branch 'upstream/v1.3.x'