Revise setPersistenceUnitName javadoc

Closes gh-36205

spring-orm/src/main/java/org/springframework/orm/jpa/AbstractEntityManagerFactoryBean.java

@@ -171,10 +171,10 @@ public abstract class AbstractEntityManagerFactoryBean implements
 	}
 
 	/**
-	 * Specify the name of the EntityManagerFactory configuration.
-	 * <p>Default is none, indicating the default EntityManagerFactory
-	 * configuration. The persistence provider will throw an exception if
-	 * ambiguous EntityManager configurations are found.
+	 * Specify the name of the persistence unit configuration to use.
+	 * <p>Default is none, indicating the default persistence unit configuration.
+	 * The persistence provider will throw an exception if ambiguous persistence
+	 * unit configurations are found.
 	 * @see jakarta.persistence.Persistence#createEntityManagerFactory(String)
 	 */
 	public void setPersistenceUnitName(@Nullable String persistenceUnitName) {

spring-orm/src/main/java/org/springframework/orm/jpa/LocalContainerEntityManagerFactoryBean.java

@@ -141,9 +141,10 @@ public class LocalContainerEntityManagerFactoryBean extends AbstractEntityManage
 	}
 
 	/**
-	 * Uses the specified persistence unit name as the name of the default
-	 * persistence unit, if applicable.
-	 * <p><b>NOTE: Only applied if no external PersistenceUnitManager specified.</b>
+	 * Specify the name of the persistence unit configuration to use.
+	 * <p>Uses the specified persistence unit name as the name of the
+	 * default persistence unit, if applicable. Otherwise, it selects
+	 * among the available persistence units.
 	 * @see DefaultPersistenceUnitManager#setDefaultPersistenceUnitName
 	 */
 	@Override

spring-orm/src/main/java/org/springframework/orm/jpa/LocalEntityManagerFactoryBean.java

@@ -127,11 +127,23 @@ public class LocalEntityManagerFactoryBean extends AbstractEntityManagerFactoryB
 	 */
 	public PersistenceConfiguration getPersistenceConfiguration() {
 		if (this.configuration == null) {
-			this.configuration = new PersistenceConfiguration(getPersistenceUnitName());
+			String name = getPersistenceUnitName();
+			Assert.state(name != null, "No persistenceUnitName set");
+			this.configuration = new PersistenceConfiguration(name);
 		}
 		return this.configuration;
 	}
 
+	/**
+	 * Specify the name of the persistence unit configuration to use.
+	 * <p>Uses the specified persistence unit name as the name of the local
+	 * persistence unit built through {@link #getPersistenceConfiguration()}, if
+	 * applicable. Otherwise, it selects among the available persistence units.
+	 * <p>Note: This setter method is not meant to be used in combination with
+	 * {@link #setPersistenceConfiguration} which derives the persistence unit
+	 * name from the given {@link PersistenceConfiguration} instance instead.
+	 * @see #getPersistenceConfiguration()
+	 */
 	@Override
 	public void setPersistenceUnitName(@Nullable String persistenceUnitName) {
 		Assert.state(this.configuration == null || this.configuration.name().equals(persistenceUnitName),