Add some clarification to GLFM fixture

What does this MR do and why?

Add some clarification to GLFM fixture data for glfm_spec.txt in spec/scripts/lib/glfm/update_example_snapshots_spec.rb, explaining and illustrating how the additional keywords for examples cause the source_specification to have different values.

How to set up and validate locally

There is no change in behavior, all specs should still pass.

MR acceptance checklist

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