Document webagg in docs.

Our docs build already depends on tornado (via ipywidgets, to build the
nbagg docs).

Author: anntzer

doc/api/backend_webagg_api.rst

@@ -1,8 +1,7 @@
 :mod:`.backend_webagg`
 ======================
 
-.. note::
-   The WebAgg backend is not documented here, in order to avoid adding Tornado
-   to the doc build requirements.
-
-.. module:: matplotlib.backends.backend_webagg
+.. automodule:: matplotlib.backends.backend_webagg
+   :members:
+   :undoc-members:
+   :show-inheritance:

doc/conf.py

@@ -161,6 +161,7 @@ def _check_dependencies():
     'pytest': ('https://pytest.org/en/stable/', None),
     'python': ('https://docs.python.org/3/', None),
     'scipy': ('https://docs.scipy.org/doc/scipy/reference/', None),
+    'tornado': ('https://www.tornadoweb.org/en/stable/', None),
 }