[docs/api] Directives documentations start with the ancestor class' description.

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Directives which extend `Directive` class has the same

> Base class for creating custom directives. Users should extend this class, implement render and/or update, and then pass their subclass to directive.

description.

Directives which extend `AsyncDirective` class has the same

> An abstract `Directive` base class whose `disconnected` method will be called when the part containing the directive is cleared as a result of re-rendering, or when the user calls `part.setConnected(false)` on a part that was previously rendered containing the directive (as happens when e.g. a LitElement disconnects from the DOM).

description

[Directives page](https://lit.dev/docs/api/directives/)

Open the linked Directives API page and compare the generated descriptions for Directive and AsyncDirective with their documented subclasses. Trace how those descriptions are sourced, then update the documentation so each subclass starts with its own description rather than the ancestor text. Done means the Directives page shows the correct descriptions for both classes.

Written by the indexing model from the issue text.