Document that rolling clang just got harder.

DEPS

 vars = {
   # Use this googlecode_url variable only if there is an internal mirror for it.
   # If you do not know, use the full path while defining your new deps entry.
   "googlecode_url": "http://%s.googlecode.com/svn",
   "webkit_trunk": "http://svn.webkit.org/repository/webkit/trunk",
   "nacl_trunk": "http://src.chromium.org/native_client/trunk",
   "webkit_revision": "93861",
   "chromium_git": "http://git.chromium.org/git",
   "swig_revision": "69281",
   "nacl_revision": "6539",
@@ skipping 17 matching lines @@
   "libjingle_revision": "77",
   "libvpx_revision": "97420",
   "ffmpeg_revision": "98072",
   "sfntly_revision": "54",
   "skia_revision": "2169",
   "v8_revision": "9008",
   "webrtc_revision": "242",
 
   # Note: On most bots, clang is not checked out via DEPS but by
   # tools/clang/scripts/update.sh. The script reads this line here.
+  # Do NOT CHANGE this if you don't know what you're doing -- see
+  # http://code.google.com/p/chromium/wiki/UpdatingClang
   "clang_revision": "138500",
 }
 
 deps = {
   "src/breakpad/src":
     (Var("googlecode_url") % "google-breakpad") + "/trunk/src@826",
 
   "src/build/util/support":
     "/trunk/deps/support@20411",
 
@@ skipping 368 matching lines @@
          "--file-hash", "linux_x86_newlib",
              Var("nacl_toolchain_linux_x86_newlib_hash"),
     ],
   },
   {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     "pattern": ".",
     "action": ["python", "src/build/gyp_chromium"],
   },
 ]