Clarify --boot-image help message

Make dex2oat print the same help as e.g. oatdump regarding
--boot-image and clarify the example, this is particularly important
as the automatic addition of the arch to the path is quite
counterintuitive.

Change-Id: I234b498d5deca99bb988a9c24ff9febc6349eb9e
diff --git a/cmdline/cmdline.h b/cmdline/cmdline.h
index 2e9f208..4aced5b 100644
--- a/cmdline/cmdline.h
+++ b/cmdline/cmdline.h
@@ -196,6 +196,7 @@
         "  --boot-image=<file.art>: provide the image location for the boot class path.\n"
         "      Do not include the arch as part of the name, it is added automatically.\n"
         "      Example: --boot-image=/system/framework/boot.art\n"
+        "               (specifies /system/framework/<arch>/boot.art as the image file)\n"
         "\n";
     usage += StringPrintf(  // Optional.
         "  --instruction-set=(arm|arm64|mips|mips64|x86|x86_64): for locating the image\n"
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index a1485e4..2aa4085 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -215,7 +215,9 @@
   UsageError("      Example: --base=0x50000000");
   UsageError("");
   UsageError("  --boot-image=<file.art>: provide the image file for the boot class path.");
+  UsageError("      Do not include the arch as part of the name, it is added automatically.");
   UsageError("      Example: --boot-image=/system/framework/boot.art");
+  UsageError("               (specifies /system/framework/<arch>/boot.art as the image file)");
   UsageError("      Default: $ANDROID_ROOT/system/framework/boot.art");
   UsageError("");
   UsageError("  --android-root=<path>: used to locate libraries for portable linking.");