Merge from Widevine repo of http://go/wvgerrit/23482
Now that the documentation has been approved, it's time to
copy text from http://go/oemcrypto to OEMCryptoCENC.h
This also has all the little tweaks to the documents that have been
requested over the past couple of weeks.
b/31458046
Change-Id: I5b865031eb7945005c06bf1b00ecfaaedcad15f4
3a9c9ce498