Auto merge of #13436 - ehuss:clarify-build-script-metadata, r=epage

docs: Minor clarification of build script metadata.

This adds a minor clarification to the build script docs around links metadata. "As mentioned above in the output format" didn't really clue me in to what section it was talking about, or clearly tell me how to set the metadata.

src/doc/src/reference/build-scripts.md

@@ -376,14 +376,16 @@ In other words, it is forbidden to have two packages link to the same native
 library. This helps prevent duplicate symbols between crates. Note, however,
 that there are [conventions in place](#-sys-packages) to alleviate this.
 
-As mentioned above in the output format, each build script can generate an
-arbitrary set of metadata in the form of key-value pairs. This metadata is
-passed to the build scripts of **dependent** packages. For example, if the
-package `bar` depends on `foo`, then if `foo` generates `key=value` as part of
-its build script metadata, then the build script of `bar` will have the
-environment variables `DEP_FOO_KEY=value`. See the ["Using another `sys`
-crate"][using-another-sys] for an example of
-how this can be used.
+Build scripts can generate an arbitrary set of metadata in the form of
+key-value pairs. This metadata is set with the `cargo::metadata=KEY=VALUE`
+instruction.
+
+The metadata is passed to the build scripts of **dependent** packages. For
+example, if the package `bar` depends on `foo`, then if `foo` generates
+`key=value` as part of its build script metadata, then the build script of
+`bar` will have the environment variables `DEP_FOO_KEY=value`. See the ["Using
+another `sys` crate"][using-another-sys] for an example of how this can be
+used.
 
 Note that metadata is only passed to immediate dependents, not transitive
 dependents.