FF131 document.caretPositionFromPoint() - support for shadow DOM (#35911

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* FF131 document.caretPositionFromPoint() - support for shadow DOM

* Apply suggestions from code review

Co-authored-by: Brian Thomas Smith <brian@smith.berlin>

* Make the if statements bullets

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Co-authored-by: Brian Thomas Smith <brian@smith.berlin>

files/en-us/web/api/caretposition/index.md

@@ -7,7 +7,8 @@ browser-compat: api.CaretPosition
 
 {{ APIRef("CSSOM") }}
 
-The `CaretPosition` interface represents the caret position, an indicator for the text insertion point. You can get a `CaretPosition` using the {{domxref("Document.caretPositionFromPoint()")}} method.
+The `CaretPosition` interface represents the caret position, an indicator for the text insertion point.
+You can get a `CaretPosition` using the {{domxref("Document.caretPositionFromPoint()")}} method.
 
 ## Instance properties
 
@@ -16,7 +17,8 @@ _This interface doesn't inherit any properties._
 - {{domxref("CaretPosition.offsetNode")}} {{ReadOnlyInline}}
   - : Returns a {{domxref("Node")}} containing the found node at the caret's position.
 - {{domxref("CaretPosition.offset")}} {{ReadOnlyInline}}
-  - : Returns a `long` representing the character offset in the caret position node.
+  - : Returns a `long` representing the offset of the selection in the caret position node.
+    This will be the character offset in a text node or the selected child node's index in an element node.
 
 ## Instance methods