Add user docs for @Module.subcomponents

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132470435

Author: ronshapiro

core/src/main/java/dagger/Component.java

@@ -171,9 +171,13 @@
  * inherit the <em>entire</em> binding graph from its parent when it is declared. For that reason,
  * a subcomponent isn't evaluated for completeness until it is associated with a parent.
  *
- * <p>Subcomponents are declared via a factory method on a parent component or subcomponent. The
- * method may have any name, but must return the subcomponent. The factory method's parameters may
- * be any number of the subcomponent's modules, but must at least include those without visible
+ * <p>Subcomponents are declared by listing the class in the {@link Module#subcomponents()}
+ * attribute of one of the parent component's modules. This binds the {@link Subcomponent.Builder}
+ * within the parent component.
+ *
+ * <p>Subcomponents may also be declared via a factory method on a parent component or subcomponent.
+ * The method may have any name, but must return the subcomponent. The factory method's parameters
+ * may be any number of the subcomponent's modules, but must at least include those without visible
  * no-arg constructors. The following is an example of a factory method that creates a
  * request-scoped subcomponent from a singleton-scoped parent: <pre><code>
  *   {@literal @}Singleton {@literal @}Component

core/src/main/java/dagger/Module.java

@@ -40,6 +40,8 @@
    * Any {@link Subcomponent}- or {@code @ProductionSubcomponent}-annotated classes which should be
    * children of the component in which this module is installed. A subcomponent may be listed in
    * more than one module in a component.
+   *
+   * @since 2.7
    */
   @Beta
   Class<?>[] subcomponents() default {};

producers/src/main/java/dagger/producers/ProducerModule.java

@@ -46,6 +46,8 @@
    * Any {@link dagger.Subcomponent}- or {@link ProductionSubcomponent}-annotated classes which
    * should be children of the component in which this module is installed. A subcomponent may be
    * listed in more than one module in a component.
+   *
+   * @since 2.7
    */
   Class<?>[] subcomponents() default {};
 }