firestore: add beta notice in godoc
pongad@ has already made this change to the gapics, and the only manual that
is non-stable is firestore. So, this CL...
Fixes
Package firestore provides a client for reading and writing to a Cloud Firestore
database.
+ NOTE: This package is in beta. It is not stable, and may be subject to changes.
+
See https://cloud.google.com/firestore/docs for an introduction
to Cloud Firestore and additional help on using the Firestore API.
