Document the `--debuggable` option in dex2oat's usage.

Change-Id: I88accd4c4b6340b67407d948c72b595349b2eede
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index 07cf88c..f9520be 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -309,6 +309,8 @@
   UsageError("      stripped using standard command line tools such as strip or objcopy.");
   UsageError("      (enabled by default in debug builds, disabled by default otherwise)");
   UsageError("");
+  UsageError("  --debuggable: Produce debuggable code. Implies --generate-debug-info.");
+  UsageError("");
   UsageError("  --no-generate-debug-info: Do not generate debug information for native debugging.");
   UsageError("");
   UsageError("  --runtime-arg <argument>: used to specify various arguments for the runtime,");