doc: Add "extensions" to usage guide

Signed-off-by: Stephen Finucane <stephen@that.guru>

doc/usage/extensions/coverage.rst

@@ -0,0 +1,42 @@
+:mod:`sphinx.ext.coverage` -- Collect doc coverage stats
+========================================================
+
+.. module:: sphinx.ext.coverage
+   :synopsis: Check Python modules and C API for coverage in the documentation.
+
+This extension features one additional builder, the :class:`CoverageBuilder`.
+
+.. class:: CoverageBuilder
+
+   To use this builder, activate the coverage extension in your configuration
+   file and give ``-b coverage`` on the command line.
+
+.. todo:: Write this section.
+
+Several new configuration values can be used to specify what the builder
+should check:
+
+.. confval:: coverage_ignore_modules
+
+.. confval:: coverage_ignore_functions
+
+.. confval:: coverage_ignore_classes
+
+.. confval:: coverage_c_path
+
+.. confval:: coverage_c_regexes
+
+.. confval:: coverage_ignore_c_items
+
+.. confval:: coverage_write_headline
+
+   Set to ``False`` to not write headlines.
+
+   .. versionadded:: 1.1
+
+.. confval:: coverage_skip_undoc_in_source
+
+   Skip objects that are not documented in the source with a docstring.
+   ``False`` by default.
+
+   .. versionadded:: 1.1