Make ACP a requirement for certain changes

Author: oskgo

src/development/feature-lifecycle.md

@@ -23,7 +23,7 @@ be merged and pass its eventual FCP.
 
 You can create an ACP in the `rust-lang/libs-team` repo using [this issue template](https://github.com/rust-lang/libs-team/issues/new?assignees=&labels=api-change-proposal%2C+T-libs-api&template=api-change-proposal.md&title=%28My+API+Change+Proposal%29). This should include a sketch of the proposed API, but does not have to be the final design that will be implemented.
 
-Note that an ACP is not strictly required: you can just go ahead and submit a pull request with an implementation of your proposed API, with the risk of wasted effort if the library team ends up rejecting this feature. However do note that this risk is always present even if an ACP is accepted, as the library team can end up rejecting a feature in the later parts of the stabilization process.
+An ACP is always required for any change to user-obvious API, and strongly recommended for any change to API. When an ACP is not necessary you can just go ahead and submit a pull request with your proposed implementation, with the risk of wasted effort if the library team ends up rejecting this feature. However do note that this risk is always present even if an ACP is accepted, as the library team can end up rejecting a feature in the later parts of the stabilization process.
 
 ## API design exploration