Trivial operators - improve documentation

[MargaretDuff]

Changes

Improves the documentation for IdentityOperator, MatrixOperator and ZeroOperator.

Testing you performed

Please add any demo scripts to https://github.com/TomographicImaging/CIL-Demos/tree/main/misc

Related issues/links

Closes #1950

Checklist

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have updated the relevant documentation
  [ ] I have implemented unit tests that cover any new or modified functionality
  [ ] CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License
• I confirm that the contribution does not violate any intellectual property rights of third parties

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[lauramurgatroyd]

Great improvement but I had extra ideas/query

[lauramurgatroyd]

thanks for the updates!