Make the link between Guest and the "Annotator" interface clearer

`Annotator` is not a great name for this interface, but at least make
the link between it and Guest clearer by improving its documentation and
marking Guest as explicitly implementing it.

src/annotator/guest.js

@@ -18,6 +18,7 @@ import { normalizeURI } from './util/url';
 
 /**
  * @typedef {import('../types/annotator').AnnotationData} AnnotationData
+ * @typedef {import('../types/annotator').Annotator} Annotator
  * @typedef {import('../types/annotator').Anchor} Anchor
  * @typedef {import('../types/annotator').Destroyable} Destroyable
  * @typedef {import('../types/annotator').SidebarLayout} SidebarLayout
@@ -101,7 +102,8 @@ function resolveAnchor(anchor) {
  * The anchoring implementation defaults to a generic one for HTML documents and
  * can be overridden to handle different document types.
  *
- * @implements Destroyable
+ * @implements {Annotator}
+ * @implements {Destroyable}
  */
 export default class Guest {
   /**

src/types/annotator.js

@@ -65,12 +65,13 @@
  */
 
 /**
- * Subset of the annotator `Guest` instance that is exposed to other modules
+ * Subset of the `Guest` class that is exposed to integrations.
  *
  * @typedef Annotator
  * @prop {Anchor[]} anchors
  * @prop {(ann: AnnotationData) => Promise<Anchor[]>} anchor
- * @prop {(frame: HTMLIFrameElement) => void} injectClient
+ * @prop {(frame: HTMLIFrameElement) => void} injectClient - Inject the Hypothesis client
+ *   into a same-origin iframe in guest mode.
  */
 
 /**