Rollup merge of #144676 - tiif:dev_guide_unstable_bound, r=BoxyUwU

Add documentation for unstable_feature_bound

There is more detail and explanation in https://hackmd.io/``````@tiif/Byd3mq7Ige``````

Original PR that implemented this: rust-lang/rust#140399

r? ``````@BoxyUwU`````` to nominate for types team discussion

Author: Kobzol