Clarification to ARF/RMF/SRM docs

[KriSun95]

Some points to clarify on the response docs:

Comments from @settwi:

• What does “efficiency” mean the first time you write it?
• It would be great to put uncertainties on the different ARG elements but that’s hard. Monte Carlo can help with that.
• Is it valid to say all of the ARF elements are diagonal? that’s implicit in showing them as a 1D curve
• Putting “counts” on the X axis is not sitting well with me. i know it is convention, but i think your documentation should go against convention.
• Spectral inversio

Comments from broader Sunkit-spex team:

• How does the "DRM" fit into this?

[settwi]

I meant to write “Spectral inversion happens with X-rays?” also ARF not ARG :p