adding some javadoc to FormatValidator#validate()

Author: erosb

core/src/main/java/org/everit/json/schema/FormatValidator.java

@@ -74,6 +74,17 @@ static FormatValidator forFormat(final String formatName) {
     }
   }
 
+  /**
+   * Implementation-specific validation of {@code subject}. If a validation error occurs then
+   * implementations should return a programmer-friendly error message as a String wrapped in an
+   * Optional. If the validation succeeded then {@link Optional#empty() an empty optional} should be
+   * returned.
+   *
+   * @param subject
+   *          the string to be validated
+   * @return an {@code Optional} wrapping the error message if a validation error occured, otherwise
+   *         {@link Optional#empty() an empty optional}.
+   */
   Optional<String> validate(String subject);
 
 }