Clarify the difference between CARGO_CRATE_NAME and CARGO_PKG_NAME

Author: lu-zero

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

@@ -231,7 +231,7 @@ corresponding environment variable is set to the empty string, `""`.
 * `CARGO_PKG_RUST_VERSION` — The Rust version from the manifest of your package.
   Note that this is the minimum Rust version supported by the package, not the
   current Rust version.
-* `CARGO_CRATE_NAME` — The name of the crate that is currently being compiled.
+* `CARGO_CRATE_NAME` — The name of the crate that is currently being compiled. It is the name of the [Cargo target] the `-` converted to `_`, such as the name of the library, binary, example, integration test, or benchmark.
 * `CARGO_BIN_NAME` — The name of the binary that is currently being compiled (if it is a binary). This name does not include any file extension, such as `.exe`.
 * `OUT_DIR` — If the package has a build script, this is set to the folder where the build
               script should place its output. See below for more information.
@@ -255,6 +255,7 @@ corresponding environment variable is set to the empty string, `""`.
   the tests/benches. Cargo initially creates this directory but doesn't
   manage its content in any way, this is the responsibility of the test code.
 
+[Cargo target]: cargo-targets.md
 [integration test]: cargo-targets.md#integration-tests
 [`env` macro]: ../../std/macro.env.html