Add short typedoc @module descriptions

It seems this needs to be done in packages/apache-annotator; if put in
the individual packages’ index.ts files, it does not show up in the docs.
diff --git a/packages/apache-annotator/src/dom.ts b/packages/apache-annotator/src/dom.ts
index f10d64f..9ab4f65 100644
--- a/packages/apache-annotator/src/dom.ts
+++ b/packages/apache-annotator/src/dom.ts
@@ -18,4 +18,18 @@
  * under the License.
  */
 
+/**
+ * This module provides functions for handling annotations in the context of an
+ * HTML DOM; in other words, a web page.
+ *
+ * The main functionality is finding which selection of the web page a {@link https://www.w3.org/TR/2017/REC-annotation-model-20170223/#selectors
+ * | Selector} refers to; and, vice versa, describing a selection of the page as
+ * a Selector.
+ *
+ * The primary way to express selections in the web page is by {@link https://developer.mozilla.org/en-US/docs/Web/API/Range
+ * | Range} objects.
+ *
+ * @module
+ */
+
 export * from '@apache-annotator/dom';
diff --git a/packages/apache-annotator/src/selector.ts b/packages/apache-annotator/src/selector.ts
index 8e52be6..70d464e 100644
--- a/packages/apache-annotator/src/selector.ts
+++ b/packages/apache-annotator/src/selector.ts
@@ -18,4 +18,16 @@
  * under the License.
  */
 
+/**
+ * This module provides types and generic functions for handling {@link https://www.w3.org/TR/2017/REC-annotation-model-20170223/#selectors
+ * | Selector}s.
+ *
+ * Annotation tool developers should not need most of the functions contained
+ * in this module, but would instead mainly use the module made for the specific
+ * context (document type) they are dealing with. See {@link dom}, currently the
+ * only such module.
+ *
+ * @module
+ */
+
 export * from '@apache-annotator/selector';