document use of `flutter_style_todos`

We agreed to adopt this style (and I enabled the lint) but forgot to update the doc.

This fixes that!

Change-Id: I68c29471506b8621ee8e6069540f7ca7dc23138e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/337286
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
diff --git a/pkg/analyzer/doc/implementation/coding_style.md b/pkg/analyzer/doc/implementation/coding_style.md
index 71c7f27..549e3c2 100644
--- a/pkg/analyzer/doc/implementation/coding_style.md
+++ b/pkg/analyzer/doc/implementation/coding_style.md
@@ -21,9 +21,10 @@
 
 ### Lint rules
 
-- prefer_single_quotes - We don't enforce this in each of our packages yet, but
+- `flutter_style_todos` - Keeps our todo format canonical.
+- `prefer_single_quotes` - We don't enforce this in each of our packages yet, but
   we aspire to.
-- unnecessary_breaks - All our packages require at least Dart `3.0.0`, so we
+- `unnecessary_breaks` - All our packages require at least Dart `3.0.0`, so we
   can do it.
 
 ### Formatting and sorting