Warnings from building documentation with sphinx

While running `python etstool.py api-docs`, sphinx emits the following warnings:

sphinx build output

```
sphinx-build -b html -d build/doctrees source build/html
Running Sphinx v2.3.1
making output directory... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 332 source files that are out of date
updating environment: [new config] 332 added, 0 changed, 0 removed
reading sources... [100%] toolkits
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/action/action_controller.py:docstring of pyface.action.action_controller.ActionController.can_add_to_menu:6: WARNING: Inline literal start-string without end-string.
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/action/action_controller.py:docstring of pyface.action.action_controller.ActionController.can_add_to_toolbar:6: WARNING: Inline literal start-string without end-string.
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/application.py:docstring of pyface.application:6: WARNING: Unknown interpreted text role "py:method".
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/application.py:docstring of pyface.application:6: WARNING: Unknown interpreted text role "py:method".
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/application.py:docstring of pyface.application:6: WARNING: Unknown interpreted text role "py:method".
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/base_toolkit.py:docstring of pyface.base_toolkit:42: WARNING: Unknown interpreted text role "py_func".
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/i_application_window.py:docstring of pyface.i_application_window.IApplicationWindow:7: WARNING: Unexpected section title.

Usage
-----
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/i_widget.py:docstring of pyface.i_widget.IWidget.show:4: WARNING: Unexpected section title.

Parameter
---------
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/i_widget.py:docstring of pyface.i_widget.IWidget.enable:4: WARNING: Unexpected section title.

Parameter
---------
WARNING: autodoc: failed to import module 'api' from module 'pyface.preference'; the following exception was raised:
cannot import name 'PreferenceDialog'
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/tasks/i_editor_area_pane.py:docstring of pyface.tasks.i_editor_area_pane.IEditorAreaPane.register_factory:5: WARNING: Definition list ends without a blank line; unexpected unindent.
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/tasks/i_editor_area_pane.py:docstring of pyface.tasks.i_editor_area_pane.IEditorAreaPane.register_factory:9: WARNING: Definition list ends without a blank line; unexpected unindent.
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/tasks/tasks_application.py:docstring of pyface.tasks.tasks_application.TaskFactory.factory:3: WARNING: Inline strong start-string without end-string.
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/timer/do_later.py:docstring of pyface.timer.do_later.do_later:5: WARNING: Inline strong start-string without end-string.
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/timer/do_later.py:docstring of pyface.timer.do_later.do_after:7: WARNING: Inline strong start-string without end-string.
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/toolkit.py:docstring of pyface.toolkit:3: WARNING: Unknown interpreted text role "py_func".
/Users/kchoi/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface-7.0.1.dev4-py3.6.egg/pyface/ui_traits.py:docstring of pyface.ui_traits.HasMargin.get_default_value:3: WARNING: Definition list ends without a blank line; unexpected unindent.
/Users/kchoi/Work/ETS/pyface/docs/source/toolkits.rst:65: WARNING: Unknown interpreted text role "py:exception".
looking for now-outdated files... none found
pickling environment... done
checking consistency... /Users/kchoi/Work/ETS/pyface/docs/source/api/modules.rst: WARNING: document isn't included in any toctree
done
preparing documents... done
writing output... [100%] toolkits
generating indices... genindex py-modindexdone
writing additional pages... searchdone
copying static files... ... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 19 warnings.

The HTML pages are in build/html.

Build finished. The HTML pages are in build/html.
```