docs: clarify where package metadata comes from

Author: mathstuf

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

@@ -61,9 +61,9 @@ let version = env!("CARGO_PKG_VERSION");
 * `CARGO_PKG_VERSION_PRE` - The pre-release version of your package.
 * `CARGO_PKG_AUTHORS` - Colon separated list of authors from the manifest of your package.
 * `CARGO_PKG_NAME` - The name of your package.
-* `CARGO_PKG_DESCRIPTION` - The description of your package.
-* `CARGO_PKG_HOMEPAGE` - The home page of your package.
-* `CARGO_PKG_REPOSITORY` - The repository of your package.
+* `CARGO_PKG_DESCRIPTION` - The description from the manifest of your package.
+* `CARGO_PKG_HOMEPAGE` - The home page from the manifest of your package.
+* `CARGO_PKG_REPOSITORY` - The repository from the manifest of your package.
 * `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.