Merge pull request #4185 from 9names/update_rp_rt_feature_docs

Clarify embassy-rp rt feature purpose

embassy-rp/Cargo.toml

@@ -26,7 +26,10 @@ features = ["defmt", "unstable-pac", "time-driver", "rp2040"]
 
 [features]
 default = [ "rt" ]
-## Enable the rt feature of [`rp-pac`](https://docs.rs/rp-pac). This brings in the [`cortex-m-rt`](https://docs.rs/cortex-m-rt) crate, which adds startup code and minimal runtime initialization.
+
+## Enable the `rt` feature of [`rp-pac`](https://docs.rs/rp-pac).
+## With `rt` enabled the PAC provides interrupt vectors instead of letting [`cortex-m-rt`](https://docs.rs/cortex-m-rt) do that.
+## See <https://docs.rs/cortex-m-rt/latest/cortex_m_rt/#device> for more info.
 rt = [ "rp-pac/rt" ]
 
 ## Enable [defmt support](https://docs.rs/defmt) and enables `defmt` debug-log messages and formatting in embassy drivers.