docs(ref): Rename DEP_NAME_KEY to DEP_LINKS_KEY (#16205)

### What does this PR try to resolve?

`NAME` is unclear what should go here and the docs have to specifically
call out that it is `package.links` and not `package.name`. The hope
with this rename is to make this even clearer, especially if we move
forward with #3544 and add an alternative to `DEP_NAME_KEY` that uses
the dependency name.

### How to test and review this PR?

Author: weihanglo

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

@@ -445,7 +445,7 @@ The metadata is passed to the build scripts of **dependent** packages. For
 example, if the package `foo` depends on `bar`, which links `baz`, then if 
 `bar` generates `key=value` as part of its build script metadata, then the
 build script of `foo` will have the environment variables `DEP_BAZ_KEY=value`
-(note that the value of the `links` key is used).
+(note that the value of the `links` key is used and the case change for `key`).
 See the ["Using another `sys` crate"][using-another-sys] for an example of 
 how this can be used.
 
@@ -475,7 +475,7 @@ convention of native-library-related packages:
 
 * Common dependencies on `foo-sys` alleviates the rule about one package per
   value of `links`.
-* Other `-sys` packages can take advantage of the `DEP_NAME_KEY=value`
+* Other `-sys` packages can take advantage of the `DEP_LINKS_KEY=value`
   environment variables to better integrate with other packages. See the
   ["Using another `sys` crate"][using-another-sys] example.
 * A common dependency allows centralizing logic on discovering `libfoo` itself

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

@@ -383,7 +383,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
   [`release`] profile. Using this environment variable is not recommended.
   Using other environment variables like `OPT_LEVEL` provide a more correct
   view of the actual settings being used.
-* `DEP_<name>_<key>` --- For more information about this set of environment variables,
+* `DEP_<links>_<key>` --- For more information about this set of environment variables,
   see build script documentation about [`links`][links].
 * `RUSTC`, `RUSTDOC` --- the compiler and documentation generator that Cargo has
   resolved to use, passed to the build script so it might use it as well.