Bug 1609662: part 5) Update documentation of `Selection::FindInsertionPoint`. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D60129

--HG--
extra : moz-landing-system : lando

dom/base/Selection.h

@@ -736,6 +736,8 @@ class Selection final : public nsSupportsWeakReference,
 
    * If there is an item in the array equal to the input point (aPointNode,
    * aPointOffset), we will return the index of this item.
+   *
+   * @param aInsertionPoint can be in [0, `aElementArray->Length()`].
    */
   static nsresult FindInsertionPoint(
       const nsTArray<RangeData>* aElementArray, const nsINode* aPointNode,