Improve doc of custom codec

- Correct parameter interface from `Parameter` to `MySqlParameter`
- Override `toString()` in the implementation of `MySqlParameter`

README.md

@@ -455,7 +455,7 @@ public final class JacksonCodec implements Codec<Object> {
         return encoding;
     }
 
-    private static final class JacksonParameter implements Parameter {
+    private static final class JacksonParameter implements MySqlParameter {
 
         private final ByteBufAllocator allocator;
 
@@ -524,6 +524,14 @@ public final class JacksonCodec implements Codec<Object> {
         public short getType() {
             return DataTypes.VARCHAR;
         }
+
+        /**
+         * Optional, for statements/parameters logging.
+         */
+        @Override
+        public String toString() {
+            return value.toString();
+        }
     }
 }
 ```

src/main/java/io/asyncer/r2dbc/mysql/codec/NullMySqlParameter.java

@@ -65,7 +65,6 @@ public void dispose() {
 
     @Override
     public String toString() {
-        // Hide parameter detail even it is null.
         return "null";
     }