Document `OVERRIDE_TARGET_FLATTEN_APEX` in ART documentation.

If one wants to forcibly enable/disable APEX flattening, they should
now use `OVERRIDE_TARGET_FLATTEN_APEX` instead of
`TARGET_FLATTEN_APEX` (which has stopped working lately).

Test: n/a (documentation-only change)
Bug: 121117762

Change-Id: I4d865eaf13e29c860e71f8c5fafad67fcd6db4b0
2 files changed