Expand RUSTUP_TOOLCHAIN_SOURCE's documentation

Author: smoelius

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

@@ -72,7 +72,7 @@
 - `RUSTUP_CONCURRENT_DOWNLOADS` *unstable* (default: the number of components to download). Controls the number of
   downloads made concurrently.
 
-- `RUSTUP_TOOLCHAIN_SOURCE` *unstable*. Set by rustup to tell proxied tools how `RUSTUP_TOOLCHAIN` was determined.
+- `RUSTUP_TOOLCHAIN_SOURCE` *unstable*. Set by rustup to tell proxied tools how `RUSTUP_TOOLCHAIN` was determined. Non-rustup tools should not set this environment variable, except insofar as to mirror an earlier invocation from rustup.
 
 [directive syntax]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/filter/struct.EnvFilter.html#directives
 [dc]: https://docs.docker.com/storage/storagedriver/overlayfs-driver/#modifying-files-or-directories