Explain that a "list" means "of lines" not of YAML

If I try

```
    - name: caching
      uses: actions/cache@v2
      with:
        path:
          - ~/.cache
        key: build-${{ github.ref }}
        restore-keys: |
          build-
```

I just get an unexplained "Startup Failure": https://github.com/spine-generic/data-multi-subject/actions/runs/742876119. Switching to a YAML multiline string

```
    - name: caching
      uses: actions/cache@v2
      with:
        path: |
          ~/.cache
        key: build-${{ github.ref }}
        restore-keys: |
          build-
```

works: https://github.com/spine-generic/data-multi-subject/actions/runs/742878382

Author: kousu

content/actions/guides/caching-dependencies-to-speed-up-workflows.md

@@ -59,7 +59,7 @@ For more information, see [`actions/cache`](https://github.com/actions/cache).
 
 - `key`: **Required** The key created when saving a cache and the key used to search for a cache. Can be any combination of variables, context values, static strings, and functions. Keys have a maximum length of 512 characters, and keys longer than the maximum length will cause the action to fail.
 - `path`: **Required** The file path on the runner to cache or restore. The path can be an absolute path or relative to the working directory.
-  - With `v2` of the `cache` action, you can specify a single path, or multiple paths as a list. Paths can be either directories or single files, and glob patterns are supported.
+  - With `v2` of the `cache` action, you can specify a single path, or multiple paths as a block of newline-separated lines. Paths can be either directories or single files, and glob patterns are supported.
   - With `v1` of the `cache` action, only a single path is supported and it must be a directory. You cannot cache a single file.
 - `restore-keys`: **Optional** An ordered list of alternative keys to use for finding the cache if no cache hit occurred for `key`.