JNI: Add docs for mockable static natives

Bug: 898261
Change-Id: I52eb3c883118b90a074542d5123557c491244ff9
Reviewed-on: https://chromium-review.googlesource.com/c/1348951
Commit-Queue: Aiden Benner <abenner@google.com>
Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613650}
1 file changed