Rewrite expanded ci-check-msrv doc so just --list works

This rewrites the newly expanded multi-line comment explaining what
the `ci-check-msrv` recipe in the `justfile` does, so that it is
still readable from top to bottom, but so that the one-line summary
is the last line.

This is needed because `just --list` only treats a single `#` line,
preceding the formal beginning of the recipe, as its documentation.

(This differs from most other uses of multi-line comments, such as
in a Rust `///` or `//!` comment, where the first line could be a
short summary, and subsequent paragraphs are regarded subordinate.)

This continues to change only the `justfile` and not to modify the
`##` comment in the corresponding `Makefile` rule.

Author: EliahKagan

justfile

@@ -238,11 +238,12 @@ cross-test-android: (cross-test 'armv7-linux-androideabi' '--no-default-features
 check-size:
     etc/check-package-size.sh
 
-# Assume the current default toolchain is the Minimal Supported Rust Version and check against it
+# This assumes the current default toolchain is the Minimal Supported Rust Version and checks
+# against it. This is run on CI in `msrv.yml`, after the MSRV toolchain is installed and set as
+# default, and after dependencies in `Cargo.lock` are downgraded to the latest MSRV-compatible
+# versions. Only if those or similar steps are done first does this work to validate the MSRV.
 #
-# This is run on CI in `msrv.yml`, after the MSRV toolchain is installed and set as default, and
-# after dependencies in `Cargo.lock` are downgraded to the latest MSRV-compatible versions.
-# Only if those or similar steps are done first does this recipe really validate the MSRV.
+# Check the MSRV, *if* the toolchain is set and `Cargo.lock` is downgraded (used on CI)
 ci-check-msrv:
     rustc --version
     cargo check -p gix