Document use of new_axis to control merge

[stephenworsley]

🚀 Pull Request

Description

Closes #6162.
Requires SciTools/iris-sample-data#33.
May need to be adapted to account for #6168.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.82%. Comparing base (3114912) to head (ff88cbc).
Report is 1 commits behind head on main.

Additional details and impacted files

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##             main    #6180   +/-   ##
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  Coverage   89.82%   89.82%           
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  Files          88       88           
  Lines       23185    23185           
  Branches     4314     4314           
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  Hits        20825    20825           
  Misses       1628     1628           
  Partials      732      732           

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[pp-mo]

The process seems very solid, but I have suggested a few possible improvements for clarity.

Obviously we aren't quite ready to merge this anyway, owing to the link with #6168 as you already noted.

[pp-mo]

Sorry, nearly there but there are a few problems remaining.

• a couple of further suggestions on the diff
• you need to include the new page in the 'further topics' TOC, since that doesn't happen automatically, and I couldn't view the built page in the docs even by substituting the page name
  I think you just need to add it here
• the docs build is now working, but the doctests are still failing. I think it's a cartopy compatibility problem, which I hope may be fixed by just updating the branch, but I'll leave you to do that in case something goes wrong

[pp-mo]

All good now, I think !