build(bazel): add comment about advancing the version of yarn used under Bazel in the future (#29431)

PR Close #29431

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@@ -48,6 +48,14 @@ node_repositories(
     node_version = "10.9.0",
     package_json = ["//:package.json"],
     preserve_symlinks = True,
+    # yarn 1.13.0 under Bazel has a regression on Windows that causes build errors on rebuilds:
+    # ```
+    # ERROR: Source forest creation failed: C:/.../fyuc5c3n/execroot/angular/external (Directory not empty)
+    # ```
+    # See https://github.com/angular/angular/pull/29431 for more information.
+    # It possible that versions of yarn past 1.13.0 do not have this issue, however, before
+    # advancing this version we need to test manually on Windows that the above error does not
+    # happen as the issue is not caught by CI.
     yarn_version = "1.12.1",
 )