Updating release docs to remind about rc argument usage.
Updating release docs to remind about rc argument usage.

Reviewed at https://reviews.apache.org/r/49642/

apache/aurora,medallia/aurora,rdelval/aurora,thinker0/aurora,apache/aurora,thinker0/aurora,crashlytics/aurora,thinker0/aurora,thinker0/aurora,apache/aurora,rdelval/aurora,medallia/aurora,crashlytics/aurora,medallia/aurora,crashlytics/aurora,apache/aurora,medallia/aurora,thinker0/aurora,rdelval/aurora,rdelval/aurora,apache/aurora,medallia/aurora,medallia/aurora,rdelval/aurora,crashlytics/aurora,crashlytics/aurora,apache/aurora,crashlytics/aurora,thinker0/aurora,rdelval/aurora