Document the default -1,-1 selection (#96245)

Clarify in the docs about what an invalid selection means, contrasted with a 0,0 selection.

packages/flutter/lib/src/services/text_input.dart

@@ -744,6 +744,9 @@ class TextEditingValue {
   ///
   /// The [text], [selection], and [composing] arguments must not be null but
   /// each have default values.
+  ///
+  /// The default value of [selection] is `TextSelection.collapsed(offset: -1)`.
+  /// This indicates that there is no selection at all.
   const TextEditingValue({
     this.text = '',
     this.selection = const TextSelection.collapsed(offset: -1),