docs(user-guide/environment-variables): clarify the unit of RUSTUP_UNPACK_RAM

Author: rami3l

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

@@ -42,7 +42,7 @@
   or to `never` to disable colors.
 
 - `RUSTUP_UNPACK_RAM` *unstable* (default: free memory or 500MiB if unable to tell, min 210MiB). Caps the amount of
-  RAM `rustup` will use for IO tasks while unpacking.
+  RAM (in bytes) `rustup` will use for IO tasks while unpacking.
 
 - `RUSTUP_NO_BACKTRACE`. Disables backtraces on non-panic errors even when
   `RUST_BACKTRACE` is set.