Flags docs: Add a reminder to `git cl format` after adding to enums.xml.

Without this, the unittests may fail if you inserted the entries arbitrarily.

Change-Id: I805029532f36358e623eda7eae6a541c3bbc62a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2546343
Auto-Submit: David Lei <dlei@google.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828712}

docs/how_to_add_your_feature_flag.md

@@ -50,7 +50,8 @@ autoninja -C out/Default unit_tests
 ./out/Default/bin/run_unit_tests --gtest_filter=AboutFlagsHistogramTest.CheckHistograms
 ```
 
-That test will ask you to add several entries to enums.xml.
+That test will ask you to add several entries to enums.xml. After doing so, run `git cl format`
+which will insert the entries in enums.xml in the correct order and run the tests again.
 You can refer to [this CL](https://chromium-review.googlesource.com/c/593707) as an example.
 
 Finally, run the following test.