Update documenation for default value of sdk_version

Follow-up to aosp/2065271. "private" is a Soong internal `SdkKind`
definition and is probably more meaningul for devs who work on
the build system. Changing it to "private platform APIs" should
hopefully make it more meaningful to a wider audience.

(Context: https://yaqs.corp.google.com/eng/q/4927173287831666688)

Test: None
Change-Id: Ied7198bb529eeaa85bc741177b414e06a7262365