Add snippet on why cache can become invalid

There have been reports of build failures due to VM updates
changing the language versions today on .com.

This documentation update aims to clarify this situation
for the future.

There are other improvements we can make:
1. Mention that the language updates can invalidate the existing caches
in the announcement blog post
2. Preemptively clear (some or all) caches just prior to update rollouts
(This, too, needs to be announced.)