Update inaccurate documentation for isUtf16Surrogate method (#60219)

packages/flutter/lib/src/painting/text_painter.dart

@@ -601,8 +601,11 @@ class TextPainter {
     canvas.drawParagraph(_paragraph, offset);
   }
 
-  // Complex glyphs can be represented by two or more UTF16 codepoints. This
-  // checks if the value represents a UTF16 glyph by itself or is a 'surrogate'.
+  // Returns true iff the given value is a valid UTF-16 surrogate. The value
+  // must be a UTF-16 code unit, meaning it must be in the range 0x0000-0xFFFF.
+  //
+  // See also:
+  //   * https://en.wikipedia.org/wiki/UTF-16#Code_points_from_U+010000_to_U+10FFFF
   static bool _isUtf16Surrogate(int value) {
     return value & 0xF800 == 0xD800;
   }