Add entrypoint hooks customization documentation.
Add entrypoint hooks customization documentation.

PiperOrigin-RevId: 435153110
Change-Id: I774cacdc3b3e238122e92522bb1cdde9e46d7611

GoogleCloudPlatform/appengine-java-standard,GoogleCloudPlatform/appengine-java-standard,GoogleCloudPlatform/appengine-java-standard,GoogleCloudPlatform/appengine-java-standard