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-context-support/src/main/java/org/springframework/cache/caffeine/package-info.java

@@ -4,6 +4,8 @@
  * allowing to set up Caffeine caches within Spring's cache abstraction.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.cache.caffeine;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-context-support/src/main/java/org/springframework/cache/ehcache/package-info.java

@@ -10,6 +10,8 @@
  * Spring's support in {@code org.springframework.cache.jcache}.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.cache.ehcache;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-context-support/src/main/java/org/springframework/cache/jcache/package-info.java

@@ -5,6 +5,8 @@
  * use in a Spring context, using a JSR-107 compliant cache provider.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.cache.jcache;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-context-support/src/main/java/org/springframework/cache/transaction/package-info.java

@@ -3,6 +3,8 @@
  * Provides synchronization of put operations with Spring-managed transactions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.cache.transaction;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-context-support/src/main/java/org/springframework/mail/javamail/package-info.java

@@ -4,6 +4,8 @@
  * class for convenient population of a JavaMail MimeMessage.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.mail.javamail;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-context-support/src/main/java/org/springframework/mail/package-info.java

@@ -3,6 +3,8 @@
  * Concrete implementations are provided in the subpackages.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.mail;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-context-support/src/main/java/org/springframework/scheduling/commonj/package-info.java

@@ -3,6 +3,8 @@
  * facility, as supported by IBM WebSphere 6.0+ and BEA WebLogic 9.0+.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.scheduling.commonj;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-context-support/src/main/java/org/springframework/scheduling/quartz/package-info.java

@@ -6,6 +6,8 @@
  * convenience classes for implementing Quartz Jobs.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.scheduling.quartz;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-context-support/src/main/java/org/springframework/ui/freemarker/package-info.java

@@ -4,6 +4,8 @@
  * within a Spring application context.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.ui.freemarker;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;