Until now, the lint only emitted a warning, when breaking public API. Now it
doesn't lint at all when the config value is not set to `false`, bringing it in
line with the other lints using this config value.
Also ensures that this config value is documented in the lint.
Correctly handle async blocks for NEEDLESS_PASS_BY_REF_MUT
Fixes https://github.com/rust-lang/rust-clippy/issues/11299.
The problem was that the `async block`s are popping a closure which we didn't go into, making it miss the mutable access to the variables.
cc `@Centri3`
changelog: none
