Add comments around runtime options "-Xgc:[no]generational_cc" processing.

These comments serve as a reminder that these options should be
accepted by the runtime as long as they are passed by
app_process/zygote.

Test: mmma art
Bug: 72446017
Bug: 120794191
Bug: 123754583
Change-Id: Ieede0536befbd9cad77c0ab9da7f553717678916