Use service type documentation as fallback for @provides

JDK-8192007 fixed the issue that `@provides` and `@uses` items on javadoc module summary pages displayed both the tag description as well as the first sentence of the service type.

The new behaviour for "uses" items is to display the `@uses' tag description if it exists, and otherwise fall back to the first sentence of the description of the service type. For "provides" items, only the description from the `@provides` tag is displayed. If no description is provided by the tag, no description is displayed.

Although displaying the first sentence of the service type description is not ideal, it is still better than to display nothing.