updated q definition

[chris-ashe]

Description

Variable descriptions have been updated and or added to give clearer definition of q

Checklist

I confirm that I have completed the following checks:

• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[timothy-nunn]

I agree with #3106 that we need to improve our definition of q in PROCESS. The presence of q and q95 makes everything a little complex.

However, I also agree for the need to carefully consider how we do this in a backwards compatible way, hence why this PR should just improve the visibility of the definition without attempting to rewrite how PROCESS handles these variables.

[mkovari]

I think "near plasma edge"is a little vague.