Local file store (Secrets and Crypto) - clarify relative path

**Describe the issue**
In the documentation, it is always referring to `[path to the JSON file]`.
While working with absolute paths works out of the box, it becomes a bit tricky when it comes to relative paths.

As I was figuring out, running with Multi-App Run template, it is relative to the given `appDirPath`.

**URL of the docs**
[https://docs.dapr.io/reference/components-reference/supported-secret-stores/file-secret-store/](https://docs.dapr.io/reference/components-reference/supported-secret-stores/file-secret-store/)

**Expected content**
Explain, which is the root path for relative files when:

- using the CLI to run a single app
- using the Multi-App Run template to run multiple apps at once