Revisit nullability annotations

This commit introduces the following changes.

1) It adds a new Spring @NonNull annotation which allows to apply
@NonNullApi semantic on a specific element, like @Nullable does.
Combined with @Nullable, it allows partial null-safety support when
package granularity is too broad.

2) @Nullable and @NonNull can apply to ElementType.TYPE_USE in order
to be used on generic type arguments (SPR-15942).

3) Annotations does not apply to ElementType.TYPE_PARAMETER anymore
since it is not supported yet (applicability for such use case is
controversial and need to be discussed).

4) @NonNullApi does not apply to ElementType.FIELD anymore since in a
lot of use cases (private, protected) it is not part for the public API
+ its usage should remain opt-in. A dedicated @NonNullFields annotation
has been added in order to set fields default to non-nullable.

5) Updated Javadoc and reference documentation.

Issue: SPR-15756

spring-orm/src/main/java/org/springframework/orm/hibernate5/package-info.java

@@ -11,6 +11,8 @@
  * <p><b>This package supports Hibernate 5.x only.</b>
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.orm.hibernate5;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-orm/src/main/java/org/springframework/orm/hibernate5/support/package-info.java

@@ -2,6 +2,8 @@
  * Classes supporting the {@code org.springframework.orm.hibernate5} package.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.orm.hibernate5.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-orm/src/main/java/org/springframework/orm/jpa/package-info.java

@@ -4,6 +4,8 @@
  * and an implementation of Spring's transaction SPI for local JPA transactions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.orm.jpa;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-orm/src/main/java/org/springframework/orm/jpa/persistenceunit/package-info.java

@@ -2,6 +2,8 @@
  * Internal support for managing JPA persistence units.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.orm.jpa.persistenceunit;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-orm/src/main/java/org/springframework/orm/jpa/support/package-info.java

@@ -2,6 +2,8 @@
  * Classes supporting the {@code org.springframework.orm.jpa} package.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.orm.jpa.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-orm/src/main/java/org/springframework/orm/jpa/vendor/package-info.java

@@ -2,6 +2,8 @@
  * Support classes for adapting to specific JPA vendors.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.orm.jpa.vendor;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-orm/src/main/java/org/springframework/orm/package-info.java

@@ -3,6 +3,8 @@
  * Contains generic DataAccessExceptions related to O/R Mapping.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.orm;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;