Help: Clarify historical-only role of CMAKE_ANDROID_API variable

Revise documentation added by commit 6b84df8da9 (Help: Document cross
compiling for Android, 2016-06-20, v3.7.0-rc1~229^2~2) to avoid implying
that `CMAKE_ANDROID_API` is interchangeable with `CMAKE_SYSTEM_VERSION`.
The latter is the authoritative value of the Android API level.

Issue: #23830