DOCS: API Tree as a term is rather technical

[silkentrance]

I think that we should rename the "API Tree" to just "Drupal API Window". The problem with "API Tree" is that it describes the nature of both the window and the way that its content is being displayed, while in fact it provides the user access to the drupal APIs, and, when using for example nbddapigen, to the APIs of third party drupal plugins as well.

This would be a sub task of #45.