Does work under .. only:: html

When I tried to use it under `..only:: html`, I got a warning and nothing was generated.

Code :

```
.. only:: html

.. automodule:: mymodule
:members:
:undoc-members:

.. automodapi:: mymodule
:inheritance-diagram:
:include-all-objects:
:no-heading:
:no-main-docstr:
```

Warning:
```
WARNING: Unknown directive type "automodapi"
```

Sphinx version: 1.8.5

Reproduce the warning with the provided Sphinx 1.8.5 example, placing the automodapi directive inside `.. only:: html`. Then inspect how the extension registers directives and verify that the example generates API documentation without an unknown-directive warning.

Written by the indexing model from the issue text.