Rollup merge of rust-lang#157958 - clubby789:doc-lint-rust-version, r=JonathanBrouwer

doc: Document `-Zlint-rust-version`

Followup to rust-lang#157707
Tracking issue: rust-lang#157574

r? JonathanBrouwer

Author: JonathanBrouwer

src/doc/unstable-book/src/compiler-flags/lint-rust-version.md

@@ -0,0 +1,9 @@
+# `lint-rust-version`
+
+The tracking issue for this feature is: [#157574](https://github.com/rust-lang/rust/issues/157574).
+
+------------------------
+
+This feature allows you to specify a minimum Rust version for the crate, which will affect lint
+emission. If following a lint suggestion would raise the MSRV above the provided value, it should
+not be emitted.