Make it clear that compile_single_file.py doesn't support Jumbo builds

compile_single_file.py doesn't support Jumbo build because compiling a
single file goes against the concept of Jumbo builds (where files get
merged together).

Change-Id: Ib53dc22ed8b61839dc936e8e20f7febbb477eff8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1657789Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>

compile_single_file.py

@@ -40,7 +40,10 @@ def main():
       required=True)
   parser.add_argument(
       '--build-dir',
-      help='The build directory, relative to the source directory.',
+      help='The build directory, relative to the source directory. Jumbo '
+           'builds aren\'t supported but you can create a non-jumbo build '
+           'config just for this script and keep using jumbo in your regular '
+           'builds.',
       required=True)
 
   options = parser.parse_args()