Mention `standalone-apex-files` in the Runtime APEX build rules.

Until ART testing and benchmarking make use of the Runtime APEX, we
have to create copies of some libraries under `/system` within the
device environment. Add a note about this in the Runtime APEX build
rules as a reminder.

Test: n/a
Bug: 121117762
Change-Id: I7d849193a782506fd9d5761b6835f72816065940
diff --git a/build/apex/Android.bp b/build/apex/Android.bp
index 7192825..15e5c02 100644
--- a/build/apex/Android.bp
+++ b/build/apex/Android.bp
@@ -12,6 +12,16 @@
     "profman",
 ]
 // - Base requirements (libraries).
+//
+// Note: ART on-device chroot-based testing and benchmarking is not yet using
+// the Runtime APEX, meaning that copies of some of these libraries have to be
+// installed in `/system` for the ART Buildbot set-up to work properly. This is
+// done by the `standalone-apex-files` Make phony target, used by the ART
+// Buildbot and Golem (see `art/Android.mk`). If you add libraries to this list,
+// you may have to also add them to `PRIVATE_RUNTIME_DEPENDENCY_LIBS` in
+// `art/Android.mk`.
+// TODO(b/121117762): Remove this note when both the ART Buildbot and Golem use
+// the Runtime APEX.
 art_runtime_base_native_shared_libs = [
     "libadbconnection",
     "libart",
@@ -116,6 +126,16 @@
 ]
 
 // Native libraries that support the core Java libraries.
+//
+// Note: ART on-device chroot-based testing and benchmarking is not yet using
+// the Runtime APEX, meaning that copies of some of these libraries have to be
+// installed in `/system` for the ART Buildbot set-up to work properly. This is
+// done by the `standalone-apex-files` Make phony target, used by the ART
+// Buildbot and Golem (see `art/Android.mk`). If you add libraries to this list,
+// you may have to also add them to `PRIVATE_RUNTIME_DEPENDENCY_LIBS` in
+// `art/Android.mk`.
+// TODO(b/121117762): Remove this note when both the ART Buildbot and Golem use
+// the Runtime APEX.
 libcore_native_shared_libs = [
     "libandroidicu",
     "libandroidio",