docs(ref): Expand on uses of Rust version

Author: epage

src/doc/src/reference/manifest.md

@@ -164,7 +164,7 @@ will have `edition` explicitly specified to a newer value.
 ### The `rust-version` field
 
 The `rust-version` field tells cargo what version of the
-Rust language and compiler you support for your package.
+Rust toolchain you support for your package.
 See [the Rust version chapter](rust-version.md) for more detail.
 
 ### The `description` field

src/doc/src/reference/rust-version.md

@@ -1,14 +1,7 @@
 # Rust Version
 
 The `rust-version` field is an optional key that tells cargo what version of the
-Rust language and compiler you support for your package.
-If the currently selected version of the Rust compiler is older than the stated
-version, cargo will exit with an error, telling the user what version is
-required.
-This affects all targets/crates in the package, including test suites,
-benchmarks, binaries, examples, etc.
-
-The `rust-version` may be ignored using the `--ignore-rust-version` option.
+Rust toolchain you support for your package.
 
 ```toml
 [package]
@@ -25,3 +18,22 @@ To find the minimum `rust-version` compatible with your project, you can use thi
 When used on packages that get published, we recommend [verifying the `rust-version`](../guide/continuous-integration.md#verifying-rust-version).
 
 > **MSRV:** Respected as of 1.56
+
+## Uses
+
+**Diagnostics:**
+
+When your package is compiled on an unsupported toolchain,
+Cargo will provide clearer diagnostics about the insufficient toolchain version rather than reporting invalid syntax or missing functionality in the standard library.
+This affects all [Cargo targets](cargo-targets.md) in the package, including binaries, examples, test suites,
+benchmarks, etc.
+
+**Development aid:**
+
+`cargo add` will auto-select the dependency's version requirement to be the latest version compatible with your `rust-version`.
+If that isn't the latest version, `cargo add` will inform users so they can make the choice on whether to keep it or update your `rust-version`.
+
+Other tools may also take advantage of it, like `cargo clippy`'s
+[`incompatible_msrv` lint](https://rust-lang.github.io/rust-clippy/stable/index.html#/incompatible_msrv).
+
+> **Note:** The `rust-version` may be ignored using the `--ignore-rust-version` option.