docs(envs): add docs for the RUSTUP_NO_RELEASE_HINT environment variable

Author: FranciscoTGouveia

doc/user-guide/src/environment-variables.md

@@ -47,6 +47,9 @@
 - `RUSTUP_NO_BACKTRACE`. Disables backtraces on non-panic errors even when
   `RUST_BACKTRACE` is set.
 
+- `RUSTUP_NO_RELEASE_HINT`. When set, suppresses the hint that is shown when a
+  new stable Rust release is (likely) available.
+
 - `RUSTUP_PERMIT_COPY_RENAME` *unstable*. When set, allows rustup to fall-back
   to copying files if attempts to `rename` result in cross-device link
   errors. These errors occur on OverlayFS, which is used by [Docker][dc]. This