Merge from Widevine repo of http://go/wvgerrit/74624
and http://go/wvgerrit/77025
Several people have asked how many keys / session are required as a
function of OEMCrypto version. That is often enough that this
information should go in the version compatibility document.
Bug: 128628354
Test: documentation change only
Change-Id: I3031013aab2feb38a0c5ce7e5304ec0d50487ea3