firestore: add an example to the comment in compareUtf8Strings() (#7113)

Author: dconeybe

firebase-firestore/src/main/java/com/google/firebase/firestore/util/Util.java

@@ -99,6 +99,22 @@ public static int compareUtf8Strings(String left, String right) {
     // used to represent code points greater than 0xFFFF which have 4-byte UTF-8 representations
     // and are lexicographically greater than the 1, 2, or 3-byte representations of code points
     // less than or equal to 0xFFFF.
+    //
+    // An example of why Case 2 is required is comparing the following two Unicode code points:
+    //
+    // |-----------------------|------------|---------------------|-----------------|
+    // | Name                  | Code Point | UTF-8 Encoding      | UTF-16 Encoding |
+    // |-----------------------|------------|---------------------|-----------------|
+    // | Replacement Character | U+FFFD     | 0xEF 0xBF 0xBD      | 0xFFFD          |
+    // | Grinning Face         | U+1F600    | 0xF0 0x9F 0x98 0x80 | 0xD83D 0xDE00   |
+    // |-----------------------|------------|---------------------|-----------------|
+    //
+    // A lexicographical comparison of the UTF-8 encodings of these code points would order
+    // "Replacement Character" _before_ "Grinning Face" because 0xEF is less than 0xF0. However, a
+    // direct comparison of the UTF-16 code units, as would be done in case 1, would erroneously
+    // produce the _opposite_ ordering, because 0xFFFD is _greater than_ 0xD83D. As it turns out,
+    // this relative ordering holds for all comparisons of UTF-16 code points requiring a surrogate
+    // pair with those that do not.
     final int length = Math.min(left.length(), right.length());
     for (int i = 0; i < length; i++) {
       final char leftChar = left.charAt(i);