Parse inheritdoc for classes

[arogachev]

What steps will reproduce the problem?

Add any PHPDoc description to a class and @inheritdoc to a subclass.

What's expected?

Render description from parent class.

What do you get instead?

Empty description.

Additional info

Q	A
Yii version	doesn't matter
Yii APIDoc version	master
PHP version	doesn't matter
Operating system	doesn't matter

Extracted from #194.

[arogachev]

Seems to be a rare case, so this is not a high priority now.