docs_builder: Add "autogenerated" tag to comment

Add an "autogenerated" tag to the Gerrit comment posted by docs_builder.
This will make it easier to filter out these comments in other tooling.

Bug: b/339023785
Change-Id: I08e51011bbe317c3791c82af5b2c8d71272733df
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/208611
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ted Pudlik <tpudlik@google.com>
diff --git a/recipes/docs_builder.py b/recipes/docs_builder.py
index d77109f..91ed8e8 100644
--- a/recipes/docs_builder.py
+++ b/recipes/docs_builder.py
@@ -147,12 +147,13 @@
     if props.comment and api.buildbucket_util.is_tryjob:
         for change in applied_changes:
             api.gerrit.set_review(
-                "post CL comment",
+                'post CL comment',
                 change.number,
                 host=change.gerrit_host,
                 message=f'Docs available at: {joined_links}',
-                notify="NONE",
+                notify='NONE',
                 ignore_automatic_attention_set_rules=True,
+                tag='autogenerated:pigweed:docs',
             )
 
     return result.RawResult(