Add note about update_build_files default behavior

Closes https://github.com/google/iree/pull/2071

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/2071 from GMNGeoffrey:submodule-update-script 7b42e6f82b5ec182a5baa3bc25d9883b9638355b
PiperOrigin-RevId: 314637558

scripts/git/update_tf_llvm_submodules.py

diff --git a/scripts/git/update_tf_llvm_submodules.py b/scripts/git/update_tf_llvm_submodules.py
index 9fdd028..9ca4e84 100755
--- a/scripts/git/update_tf_llvm_submodules.py
+++ b/scripts/git/update_tf_llvm_submodules.py
@@ -63,7 +63,8 @@
       default="TENSORFLOW")
   parser.add_argument(
       "--update_build_files",
-      help="Updates the IREE LLVM build files from TensorFlow",
+      help=("Updates the IREE LLVM build files from TensorFlow."
+            "Defaults to True iff llvm_commit==TENSORFLOW"),
       type=utils.str2bool,
       nargs="?",
       default=None)