no-slow-types - Add documentation on unsupported-expando-property

[dsherret]

• https://github.com/denoland/deno_graph/pull/428/files#diff-bfee9a1b96f39956d569df6c610e0e8b4eb5fb38ea127a03cd0abc38dea7c7e3R269
• fix(fast_check): support expando properties denoland/deno_graph#428 (comment)

Might not be worth doing though because this is just a temporary error until we can improve it.