Extend classes documentation

This should encourage developers and users of this service pick the
right tool for the right job.

service2.js

@@ -20,6 +20,12 @@ window.addEventListener('resize', () =>
  *
  * Base class for all grammar-checking services
  *
+ * This class provides properties and implementations of methods common to
+ * all types of HTML controls.
+ *
+ * This is an intermediate class and may not be used directly in client
+ * code.
+ *
  *************************************************************************/
 class BesService {
   constructor(hostElement) {
@@ -356,6 +362,14 @@ class BesService {
  *
  * Grammar-checking service base class for tree-organized editors
  *
+ * This class provides common properties and methods for HTML controls
+ * where text content is organized as a DOM tree. The grammar is checked
+ * recursively and the DOM elements in tree specify which parts of text
+ * represent same unit of text: block element => one standalone paragraph
+ *
+ * This is an intermediate class and may not be used directly in client
+ * code.
+ *
  *************************************************************************/
 class BesTreeService extends BesService {
   constructor(hostElement) {
@@ -727,6 +741,13 @@ class BesTreeService extends BesService {
  *
  * DOM grammar-checking service
  *
+ * This class provides grammar-checking functionality to contenteditable=
+ * "true" HTML controls.
+ *
+ * May also be used on most of the HTML elements to highlight grammar
+ * mistakes. Replacing text with suggestions from the grammar-checker will
+ * not be possible when contenteditable is not "true".
+ *
  *************************************************************************/
 class BesDOMService extends BesTreeService {
   constructor(hostElement) {
@@ -799,6 +820,9 @@ class BesDOMService extends BesTreeService {
  *
  * CKEditor grammar-checking service
  *
+ * This class provides grammar-checking functionality to CKEditor
+ * controls.
+ *
  *************************************************************************/
 class BesCKService extends BesTreeService {
   constructor(hostElement, ckEditorInstance) {
@@ -940,6 +964,13 @@ class BesCKService extends BesTreeService {
  *
  * Plain-text grammar-checking service
  *
+ * This class provides grammar-checking functionality to contenteditable=
+ * "plaintext-only" HTML controls.
+ *
+ * Note: Chrome and Edge only, as Firefox reverts contenteditable=
+ * "plaintext-only" to "false". The grammar mistakes will be highlighted
+ * nevertheless, but consider using BesDOMService on Firefox instead.
+ *
  *************************************************************************/
 class BesPlainTextService extends BesService {
   constructor(hostElement) {
@@ -1359,6 +1390,9 @@ class BesPlainTextService extends BesService {
  *
  * Grammar mistake popup dialog
  *
+ * This is internal class implementing the pop-up dialog user may invoke
+ * by clicking on a highlighted grammar mistake in text.
+ *
  *************************************************************************/
 class BesPopup extends HTMLElement {
   constructor() {