Improve dex2oat --dex-file and --dex-location usage info.

Change-Id: I8cb2070149ad9fdcf01d773abb1547811999772f
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index bb80a70..8512554 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -109,9 +109,14 @@
 
   UsageError("Usage: dex2oat [options]...");
   UsageError("");
-  UsageError("  --dex-file=<dex-file>: specifies a .dex file to compile.");
+  UsageError("  --dex-file=<dex-file>: specifies a .dex, .jar, or .apk file to compile.");
   UsageError("      Example: --dex-file=/system/framework/core.jar");
   UsageError("");
+  UsageError("  --dex-location=<dex-location>: specifies an alternative dex location to");
+  UsageError("      encode in the oat file for the corresponding --dex-file argument.");
+  UsageError("      Example: --dex-file=/home/build/out/system/framework/core.jar");
+  UsageError("               --dex-location=/system/framework/core.jar");
+  UsageError("");
   UsageError("  --zip-fd=<file-descriptor>: specifies a file descriptor of a zip file");
   UsageError("      containing a classes.dex file to compile.");
   UsageError("      Example: --zip-fd=5");