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
CMake now supports cross compiling for Android using the NDK or a
standalone toolchain. Document the associated variables and how how to
write toolchain files for Android.