Diskstation widget: Amend setup instructions in docs

[benzimmer]

Proposed change

At least for my installation it was also necessary to give the new user permission to access the "File Station" application in addition to DSM. This change reflects this discovery in the docs.

Type of change

• [ ] New service widget
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Documentation only
• [ ] Other (please explain)

Checklist:

• [x] If applicable, I have added corresponding documentation changes.
• [x] If applicable, I have reviewed the feature and / or service widget guidelines.
• [x] I have checked that all code style checks pass using pre-commit hooks and linting checks.
• [x] If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.

[shamoon]

Im not going to make this change without someone with one of these devices verifying it. Do you have that somewhere? In general we don't like to include instructions for other products in our docs for exactly this reason, we constantly get back-and-forth about what the right way is to do something. Obviously its unavoidable as in this case, to have some info here, but the current instructions have been 'working' presumably for a couple of years now.

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