docs(site): document the Dialect preset in the parser features section

Per review on #2511: the usage page's parser features section gets the
withDialect preset paragraph and example, which also covers
withHashLineComments (part of the MySQL and MariaDB presets, previously
undocumented).

Author: fudianchn

src/site/sphinx/usage.rst

@@ -317,4 +317,16 @@ Additionally there are Features to control the Parser's effort at the cost of th
             sqlStr
             , parser -> parser
                 .withBackslashEscapeCharacter(true)
-    );
+    );
+
+Instead of turning the individual Parser Features on one by one, a ``Dialect`` preset selects the features of that database dialect: ``withDialect(Dialect.MYSQL)`` turns on ``withBackslashEscapeCharacter`` and ``withHashLineComments`` (both MySQL and MariaDB syntax), ``withDialect(Dialect.SQLSERVER)`` turns on ``withSquareBracketQuotation``. Features set explicitly after the dialect preset win over the preset.
+
+.. code-block:: java
+
+    // Select the Database Dialect: turns on that dialect's parser features
+    sqlStr="SELECT `col` FROM t WHERE a = 'x\\'yz' AND b = 42#24";
+    Statement stmt3 = CCJSqlParserUtil.parse(
+            sqlStr
+            , parser -> parser
+                .withDialect(Dialect.MYSQL)
+    );