Mention unsupported set syntax
Mention unsupported set syntax

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PiperOrigin-RevId: 143453922
MOS_MIGRATED_REVID=143453922

bazelbuild/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website,bazelbuild/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website