docs: make it clearer that rust_version is enforced during compile

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.

Author: ericswpark

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

@@ -24,6 +24,9 @@ Cargo will provide clearer diagnostics about the insufficient toolchain version
 This affects all [Cargo targets](cargo-targets.md) in the package, including binaries, examples, test suites,
 benchmarks, etc.
 
+Cargo will not build your package on an unsupported toolchain, unless the `--ignore-rust-version` option
+below is specified.
+
 **Development aid:**
 
 `cargo add` will auto-select the dependency's version requirement to be the latest version compatible with your `rust-version`.