Change the javadoc of Correspondence and fuzzy.md to recommend factory methods over subclassing.

The empty constructor is explicitly added just so that we can add javadoc recommending against it. (We could deprecate it, but I don't think we want to go that far, and anyway I think it probably wouldn't do much good as folks shouldn't be explicitly calling it.)

Also includes some minor improvements such as adding more links to fuzzy.md and reordering the code in the examples in javadoc more logically.

RELNOTES=The officially recommended way of creating Correspondence instances is now to use the factory methods, rather than subclassing.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=238501597

Author: peteg

core/src/main/java/com/google/common/truth/Correspondence.java

@@ -51,16 +51,21 @@
  * should have any of the other properties of an equivalence relation (reflexivity, symmetry, or
  * transitivity).
  *
- * <p>Subclasses may optionally override {@link #formatDiff}. This results in failure messages
- * including formatted diffs between expected and actual elements, where possible.
+ * <p>Optionally, instances of this class can also provide functionality to format the difference
+ * between values which do not correspond. This results in failure messages including formatted
+ * diffs between expected and actual value, where possible.
+ *
+ * <p>The recommended approach for creating an instance of this class is to use one of the static
+ * factory methods. The most general of these is {@link #from}; the other methods are more
+ * convenient in specific cases. The optional diff-formatting functionality can be added using
+ * {@link #formattingDiffsUsing}. (Alternatively, you can subclass this class yourself, but that is
+ * generally not recommended.)
  *
  * <p>Instances of this are typically used via {@link IterableSubject#comparingElementsUsing},
  * {@link MapSubject#comparingValuesUsing}, or {@link MultimapSubject#comparingValuesUsing}.
  *
  * @author Pete Gillin
  */
-// TODO(b/119038898): Once there is little reason to prefer subclassing to factory methods, change
-// the class-level javadoc to point folks at the factory methods and drop references to subclassing.
 public abstract class Correspondence<A, E> {
 
   /**
@@ -87,11 +92,11 @@ public abstract class Correspondence<A, E> {
    *
    * <pre>{@code
    * class MyRecordTestHelper {
+   *   static final Correspondence<MyRecord, MyRecord> EQUIVALENCE =
+   *       Correspondence.from(MyRecordTestHelper::recordsEquivalent, "is equivalent to");
    *   static boolean recordsEquivalent(@Nullable MyRecord actual, @Nullable MyRecord expected) {
    *     // code to check whether records should be considered equivalent for testing purposes
    *   }
-   *   static final Correspondence<MyRecord, MyRecord> EQUIVALENCE =
-   *       Correspondence.from(MyRecordTestHelper::recordsEquivalent, "is equivalent to");
    * }
    * }</pre>
    *
@@ -308,8 +313,12 @@ public String toString() {
     }
   }
 
-  // TODO(b/119038898): Once all functionality is available via factory methods, consider explicitly
-  // adding the no-arg constructor here, and use its javadoc to discourage subclassing.
+  /**
+   * Constructor. Creating subclasses (anonymous or otherwise) of this class is <i>not
+   * recommended</i>, but is possible via this constructor. The recommended approach is to use the
+   * factory methods instead (see {@linkplain Correspondence class-level documentation}).
+   */
+  protected Correspondence() {}
 
   /**
    * Returns a new correspondence which is like this one, except that the given formatter may be
@@ -326,15 +335,15 @@ public String toString() {
    *
    * <pre>{@code
    * class MyRecordTestHelper {
+   *   static final Correspondence<MyRecord, MyRecord> EQUIVALENCE =
+   *       Correspondence.from(MyRecordTestHelper::recordsEquivalent, "is equivalent to")
+   *           .formattingDiffsUsing(MyRecordTestHelper::formatRecordDiff);
    *   static boolean recordsEquivalent(@Nullable MyRecord actual, @Nullable MyRecord expected) {
    *     // code to check whether records should be considered equivalent for testing purposes
    *   }
    *   static String formatRecordDiff(@Nullable MyRecord actual, @Nullable MyRecord expected) {
    *     // code to format the diff between the records
    *   }
-   *   static final Correspondence<MyRecord, MyRecord> EQUIVALENCE =
-   *       Correspondence.from(MyRecordTestHelper::recordsEquivalent, "is equivalent to")
-   *           .formattingDiffsUsing(MyRecordTestHelper::formatRecordDiff);
    * }
    * }</pre>
    */
@@ -685,7 +694,8 @@ final boolean safeCompare(
    * expected} values, if possible, or {@code null} if not.
    *
    * <p>The implementation on the {@link Correspondence} base class always returns {@code null}. To
-   * enable diffing, subclasses should override this method.
+   * enable diffing, use {@link #formattingDiffsUsing} (or override this method in a subclass, but
+   * factory methods are recommended over subclassing).
    *
    * <p>Assertions should only invoke this with parameters for which {@link #compare} returns {@code
    * false}.