Add further details to documentation on ISIS SANS scaled background subtraction

[rbauststfc]

Description of work

Summary of work

Updates the ISIS SANS interface scaled background subtraction documentation to provide greater detail on how it currently works when time slicing.

Refs #38087.

Further detail of work

We plan to review and make some changes to the current functionality for performing a scaled background subtraction with a time sliced sample in the ISIS SANS GUI (see referenced issue for more details). In the meantime, this PR updates the documentation to ensure that users understand fully how the existing functionality is working.

To test:

Build the docs-html target and check that the changes render correctly and are understandable.

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

---

Reviewer

Please comment on the points listed below (full description).
Your comments will be used as part of the gatekeeper process, so please comment clearly on what you have checked during your review. If changes are made to the PR during the review process then your final comment will be the most important for gatekeepers. In this comment you should make it clear why any earlier review is still valid, or confirm that all requested changes have been addressed.

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
• If there is GUI work does it follow the GUI standards?
• If there are changes in the release notes then do they describe the changes appropriately?
• Do the release notes conform to the release notes guide?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

Does everything look good? Mark the review as Approve. A member of @mantidproject/gatekeepers will take care of it.

Gatekeeper

If you need to request changes to a PR then please add a comment and set the review status to "Request changes". This will stop the PR from showing up in the list for other gatekeepers.