Mention updating prebuilts version in RenderScriptPrebuilts.md

Mention that during a RenderScript rebase, the prebuilts toolchain used
to build RenderScript tools should also be updated.

Change-Id: I19d456340293861dc0f3b0a17d55dfed8730a2a8
(cherry picked from commit a967394529ae1f039495f1102116c7377f201ef9)
diff --git a/RenderScriptPrebuilts.md b/RenderScriptPrebuilts.md
index 186a456..8f25f30 100644
--- a/RenderScriptPrebuilts.md
+++ b/RenderScriptPrebuilts.md
@@ -46,22 +46,25 @@
 Generating New Prebuilts
 ------------------------
 
-1. Iteratively attempt to build the platform and fix any API differences in
+1. Set RS_LLVM_PREBUILTS_VERSION in build/core/clang/config.mk to the latest
+   prebuilts generated for this rebase, or to the latest platform version if no
+   new prebuilts were generated.
+2. Iteratively attempt to build the platform and fix any API differences in
    frameworks/compile/slang, and/or frameworks/compile/libbcc. This may entail
    updating the various snapshots of Bitcode Readers/Writers.
-2. Update RenderScript prebuilts.
+3. Update RenderScript prebuilts.
 
         cd $ANDROID_BUILD_TOP/frameworks/rs
         ./update_rs_prebuilts.sh
 
-3. The prebuilts get copied to **prebuilts/sdk**, so we must upload the
+4. The prebuilts get copied to **prebuilts/sdk**, so we must upload the
 relevant bits from there.
 
         cd $ANDROID_BUILD_TOP/prebuilts/sdk
         git commit -a
         repo upload .
 
-4. Submit CLs.
+5. Submit CLs.
 
 
 Testing Checklist