docs(user-guide/environment-variables): update description of RUSTUP_IO_THREADS

Author: rami3l

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

@@ -28,7 +28,7 @@
 - `RUSTUP_VERSION` (default: none). Overrides the rustup version (e.g. `1.27.1`)
   to be downloaded when executing `rustup-init.sh` or `rustup self update`.
 
-- `RUSTUP_IO_THREADS` *unstable* (defaults to reported cpu count). Sets the
+- `RUSTUP_IO_THREADS` *unstable* (default: reported cpu count, max 8). Sets the
   number of threads to perform close IO in. Set to `1` to force
   single-threaded IO for troubleshooting, or an arbitrary number to override
   automatic detection.