Updates to LoadISISNexus documentation

[Anders-Markvardsen]

Some smaller changes to wordings in section "Data loaded from Nexus File" of LoadISISNexus.
(needed for a sine2020 report)

To test:

• Test that changes to wordings are acceptable

Fixes #19400.

RELEASE NOTES
Does not need to be in the release notes.

---

Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards? Is it well structured with small focussed classes/methods/functions?
• Are there unit/system tests in place? Are the unit tests small and test the a class in isolation?
• If there are changes in the release notes then do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?

• How do the changes handle unexpected situations, e.g. bad input?

• Has the relevant documentation been added/updated?

• Is user-facing documentation written in a user-friendly manner?

• Has developer documentation been updated if required?

• Does everything look good? Comment with the ship it emoji but don't merge. A member of @mantidproject/gatekeepers will take care of it.

[DavidFair]

LGTM