Doc for support multitarget in config

Author: weihanglo

src/doc/src/reference/unstable.md

@@ -235,6 +235,12 @@ or running tests for both targets:
 cargo test --target x86_64-unknown-linux-gnu --target i686-unknown-linux-gnu
 ```
 
+This can also be specified in `.cargo/config.toml` files.
+
+```toml
+[build]
+target = ["x86_64-unknown-linux-gnu", "i686-unknown-linux-gnu"]
+```
 
 #### New `dir-name` attribute