Update ISIS Reflectometry GUI manual testing instructions

[rbauststfc]

Description of work

Summary of work

Updates the manual testing instructions for the ISIS Reflectometry GUI based on feedback from 6.9 release testing.

Fixes #36728.

To test:

• Build the dev-docs-html target and check that the changes render OK.
• Check that the changes address the updates requested on the linked issue.

This does not require release notes because it is just a testing documentation update.

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Reviewer

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Code Review

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[cailafinn]

Reviewed changes in both the .rst code and in the built HTML, formatting and readability all looks right. Instructions are clear and can be followed on the GUI easily.

Great to have these in for the next release. Nice.