Correct the default value of nestedTransactionAllowed in Javadoc

[Dockerel]

Correct the default value of nestedTransactionAllowed in Javadoc

• Changed the default value in the Javadoc from false to true
• For smoother wording, replaced "though, since" to "but" and “use” with “disallow”

The Javadoc for JpaTransactionManager currently states that the nestedTransactionAllowed flag defaults to false. However, the default constructor has been setting this flag to true.

/**
* Create a new JpaTransactionManager instance.
* <p>An EntityManagerFactory has to be set to be able to use it.
* @see #setEntityManagerFactory
*/
public JpaTransactionManager() {
    setNestedTransactionAllowed(true);
}

This PR updates the documentation to align with the actual behavior, ensuring consistency between the code and its Javadoc.

Let me know if you have any concerns or suggestions for further improvement.
Thank you for your time and consideration!