docs: Make it clear that CARGO_CFG_TARGET_FAMILY is multi-valued

Author: madsmtm

src/doc/src/reference/environment-variables.md

@@ -342,7 +342,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
   those defined in `RUSTFLAGS`). Some examples of what these variables are:
     * `CARGO_CFG_UNIX` --- Set on [unix-like platforms].
     * `CARGO_CFG_WINDOWS` --- Set on [windows-like platforms].
-    * `CARGO_CFG_TARGET_FAMILY=unix` --- The [target family].
+    * `CARGO_CFG_TARGET_FAMILY=unix,wasm` --- The [target family].
     * `CARGO_CFG_TARGET_OS=macos` --- The [target operating system].
     * `CARGO_CFG_TARGET_ARCH=x86_64` --- The CPU [target architecture].
     * `CARGO_CFG_TARGET_VENDOR=apple` --- The [target vendor].