Merge "Document changes to soong config string variables" am: 56a6905d

Original change: https://android-review.googlesource.com/c/platform/build/+/2290344



Change-Id: I760ad49579961037628fde4d0300644aa2f6c188
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>