Add comment to @publicapi making it clear its only a convention for non-proguarded projects. (#263)

Author: mikelehen

firebase-common/src/main/java/com/google/firebase/annotations/PublicApi.java

@@ -21,6 +21,10 @@
 /**
  * Indicates that this object (class, method, etc) should be retained while preguarding an SDK, but
  * is safe to be proguarded away in the final app.
+ *
+ * <p>NOTE: For projects /not/ using proguard, the PublicApi annotation acts merely as a convention
+ * and has no actual impact. These projects must abide by normal java visibility rules to govern the
+ * visibility of methods in their public API.
  */
 @KeepForSdk
 @Target({ElementType.TYPE, ElementType.FIELD, ElementType.METHOD, ElementType.CONSTRUCTOR})