docs: make it clearer that rust_version is enforced during compile (#15303)

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### What does this PR try to resolve?

The current wording makes it sound like Cargo will only refuse to
compile the package if it encounters any missing functionality in stdlib
while compiling the package on an unsupported toolchain. However, cargo
will actually refuse to build the package unless overridden or the
toolchain is updated.

### How should we test and review this PR?

No tests are required or applicable for this PR unless I am mistaken

### Additional information

Author: epage

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

@@ -19,10 +19,11 @@ identifiers such as -nightly will be ignored while checking the Rust version.
 
 **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.
+When your package is compiled on an unsupported toolchain, Cargo will report that as an error to the user. This makes the support expectations clear and avoids reporting a less direct diagnostic like 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.
+A user can opt-in to an unsupported build of a package with the `--ignore-rust-version` flag.
+
 
 **Development aid:**