Recommend package.rust-version in the Rust version section of reference/semver.md. (#15806)

Adds a recommendation that, to mitigate “possibly breaking” Rust version
requirement increases, they should be declared using
`package.rust-version`.

My main goal in this change is to make it *possible to find* the Rust
version section in this page by searching it, but the most appropriate
way to fit it into this section seemed to me to be making this
recommendation. Declaring (and maintaining) `package.rust-version` is
additional work, but it is a much lesser burden on maintainers than
other items in the same list such as "Provide a large window of
support".

Author: weihanglo

src/doc/src/reference/semver.md

@@ -2018,6 +2018,10 @@ previous releases). Just keep in mind that some large projects may not be able
 to update their Rust toolchain rapidly.
 
 Mitigation strategies:
+* Document your package’s minimum-supported Rust version by setting
+  [`package.rust-version`], allowing Cargo’s dependency resolution to
+  attempt to [select older versions of your package] when needed.
+  Be sure to consider the [support expectations] when doing so.
 * Use [Cargo features] to make the new features opt-in.
 * Provide a large window of support for older releases.
 * Copy the source of new standard library items if possible so that you
@@ -2029,6 +2033,9 @@ Mitigation strategies:
   mechanism for new features. These are currently unstable and only available
   in the nightly channel.
 
+[select older versions of your package]: https://doc.rust-lang.org/cargo/reference/resolver.html#rust-version
+[support expectations]: https://doc.rust-lang.org/cargo/reference/rust-version.html#support-expectations
+
 ### Possibly-breaking: changing the platform and environment requirements {#env-change-requirements}
 
 There is a very wide range of assumptions a library makes about the
@@ -2296,6 +2303,7 @@ document what your commitments are.
 
 [`cfg` attribute]: ../../reference/conditional-compilation.md#the-cfg-attribute
 [`no_std`]: ../../reference/names/preludes.html#the-no_std-attribute
+[`package.rust-version`]: https://doc.rust-lang.org/cargo/reference/rust-version.html
 [`pub use`]: ../../reference/items/use-declarations.html
 [Cargo feature]: features.md
 [Cargo features]: features.md