Improve how the build deals with javadoc invalid references

This commit improves how the build deals with javadoc invalid references
in two ways.

Link/see references that are temporarily invalid during javadoc
generation of individual modules are better masked by using the option
`Xdoclint:syntax` instead of `Xdoclint:none` (warnings were still
visible in some cases, e.g. when individually building the javadoc for
a specific module).

Global javadoc-building task `api` now combines `syntax` and `reference`
`Xdoclint` groups, allowing to raise truly invalid references even when
all the modules have been aggregated.

This commit also fixes the 20+ errors which appeared following the later
change in doclet configuration.

Closes gh-30428

spring-jdbc/src/main/java/org/springframework/jdbc/datasource/DriverManagerDataSource.java

@@ -48,9 +48,9 @@ import org.springframework.util.ClassUtils;
  * the container. Such a DataSource can be exposed as a DataSource bean in a Spring
  * ApplicationContext via {@link org.springframework.jndi.JndiObjectFactoryBean},
  * for seamless switching to and from a local DataSource bean like this class.
- * For tests, you can then either set up a mock JNDI environment through Spring's
- * {@link org.springframework.mock.jndi.SimpleNamingContextBuilder}, or switch the
- * bean definition to a local DataSource (which is simpler and thus recommended).
+ * For tests, you can then either set up a mock JNDI environment through complete
+ * solutions from third parties such as <a href="https://github.com/h-thurow/Simple-JNDI">Simple-JNDI</a>,
+ * or switch the bean definition to a local DataSource (which is simpler and thus recommended).
  *
  * <p>This {@code DriverManagerDataSource} class was originally designed alongside
  * <a href="https://commons.apache.org/proper/commons-dbcp">Apache Commons DBCP</a>