Update documentation about `core-all-system-modules` in `JavaLibrary.bp`.

Mention that this module is also used to compile Libcore tests and ART
run-tests.

Test: n/a (comment-only change)
Bug: 147814778
Change-Id: Icf57f2093b995e944fb10edcf54966db917f33ca
diff --git a/JavaLibrary.bp b/JavaLibrary.bp
index a383da7..ab37964 100644
--- a/JavaLibrary.bp
+++ b/JavaLibrary.bp
@@ -200,7 +200,8 @@
 // It does not contain other parts of core library like conscrypt, bouncycastle,
 // etc. This system_modules definition is used to bootstrap compilation for
 // other parts of the core library like core-oj, core-libart, conscrypt,
-// bouncycastle, etc.
+// bouncycastle, etc. It is also used to compile Libcore tests, as well as ART
+// Java tests (run-tests).
 java_system_modules {
     name: "core-all-system-modules",