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

@@ -19,10 +19,12 @@ 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 provide
+clearer diagnostics about the insufficient toolchain version by always erroring
+during compilation, 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:**