Update AppSearchManager to clarify features

/**

 * Provides access to the centralized AppSearch index maintained by the system.

 *

 * <p>AppSearch is a search library for managing structured data featuring:

 * <p>AppSearch is an offline, on-device search library for managing structured data featuring:

 *

 * <ul>

 *   <li>A fully offline on-device solution

 *   <li>A set of APIs for applications to index documents and retrieve them via full-text search

 *   <li>APIs for applications to allow the System to display their content on system UI surfaces

 *   <li>Similarly, APIs for applications to allow the System to share their content with other

 *       specified applications.

 *   <li>APIs to index and retrieve data via full-text search.

 *   <li>An API for applications to explicitly grant read-access permission of their data to other

 *   applications.

 *   <b>See: {@link SetSchemaRequest.Builder#setSchemaTypeVisibilityForPackage}</b>

 *   <li>An API for applications to opt into or out of having their data displayed on System UI

 *   surfaces by the System-designated global querier.

 *   <b>See: {@link SetSchemaRequest.Builder#setSchemaTypeDisplayedBySystem}</b>

 * </ul>

 *

 * <p>Applications create a database by opening an {@link AppSearchSession}.