Cranelift is the same as Wasmtime's MSRV now, more words too

docs/contributing-coding-guidelines.md

@@ -21,13 +21,16 @@ your editor.
 
 ### Minimum Supported `rustc` Version
 
-Wasmtime supports the latest three stable releases of Rust.
-
-Cranelift supports stable Rust, and follows the [Rust Update Policy for
-Firefox].
+Wasmtime and Cranelift supports the latest three stable releases of Rust. This
+means that if the latest version of Rust is 1.72.0 then Wasmtime supports Rust
+1.70.0, 1.71.0, and 1.72.0. CI will test by default with 1.72.0 and there will
+be one job running the full test suite on Linux x86\_64 on 1.70.0.
 
 Some of the CI jobs depend on nightly Rust, for example to run rustdoc with
 nightly features, however these use pinned versions in CI that are updated
 periodically and the general repository does not depend on nightly features.
 
+Updating Wasmtime's MSRV is done by editing the `rust-version` field in the
+workspace root's `Cargo.toml`
+
 [Rust Update Policy for Firefox]: https://wiki.mozilla.org/Rust_Update_Policy_for_Firefox#Schedule