Auto merge of #9836 - Firstyear:9804-cargo-docs-improve, r=alexcrichton

Add hint for cargo metadata in environment section

Fixes #9804

Author: bors

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

@@ -378,3 +378,5 @@ Cargo exposes this environment variable to 3rd party subcommands
 (ie. programs named `cargo-foobar` placed in `$PATH`):
 
 * `CARGO` — Path to the `cargo` binary performing the build.
+
+For extended information about your environment you may run `cargo metadata`.