Improve lowercased comments documentation (swiftlang#12579)

Differentiate better between a full string character lowercased and a first character only lowercased

Author: Valpertui

stdlib/public/core/String.swift

@@ -1138,9 +1138,9 @@ extension String {
   ///
   /// Here's an example of transforming a string to all lowercase letters.
   ///
-  ///     let cafe = "Café 🍵"
+  ///     let cafe = "BBQ Café 🍵"
   ///     print(cafe.lowercased())
-  ///     // Prints "café 🍵"
+  ///     // Prints "bbq café 🍵"
   ///
   /// - Returns: A lowercase copy of the string.
   ///