Improve JCasC docs

[pmarinova]

Add note on how relative file paths are handled.

As mentioned in jenkinsci/configuration-as-code-plugin#695 and jenkinsci/configuration-as-code-plugin#1145, it is not immediately obvious how relative paths are resolved. Adding a simple note may save a considerable time for someone new to this, like me.

Testing done

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[basil]

Thanks for the PR!