Clarify context add subscription argument

[JimMadge]

:white_check_mark: Checklist

• [x] You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• [x] You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
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:vertical_traffic_light: Depends on

:arrow_heading_up: Summary

:closed_umbrella: Related issues

Closes #1611

:microscope: Tests

[github-actions[bot]]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
data_safe_haven/context
context.py
data_safe_haven/provisioning
sre_provisioning_manager.py
data_safe_haven/serialisers
context_base.py
data_safe_haven/types
annotated_types.py
data_safe_haven/validators
typer.py
validators.py
Project Total

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