Make @PreviewApi a documented annotation. (#3976)

We want methods annotated with @PreviewApi to include it in their javadoc.

Author: rlazo

firebase-annotations/src/main/java/com/google/firebase/annotations/PreviewApi.java

@@ -14,6 +14,7 @@
 
 package com.google.firebase.annotations;
 
+import java.lang.annotation.Documented;
 import java.lang.annotation.ElementType;
 import java.lang.annotation.Target;
 
@@ -23,4 +24,5 @@
  * API stability or backward-compatibility.
  */
 @Target({ElementType.TYPE, ElementType.FIELD, ElementType.METHOD, ElementType.CONSTRUCTOR})
+@Documented
 public @interface PreviewApi {}