Fix whatsnew api links.

[pp-mo]

🚀 Pull Request

Description

A really tiny improvement : I was checking out the new whatsnew format + I noticed that these API refs were subtly wrong + don't function as links.

[trexfeathers]

@pp-mo I think there might be some more too. I'm just going to build some docs from this PR, go through and make a full list...

[trexfeathers]

While here, please could you also address:

• ~iris.Cube.cube.remove_coord on L50
• L64, where trailing an s after ~iris.coords.CellMeasure appears to have prevented several words rendering

[pp-mo]

👍 for diligence.

I followed your suggestions.
I have also improved/corrected a few other formatting problems :

• changed some 'single backticks' for "code element" formatting (i.e. not external links) to ''double backticks''
  • (NB backticks confuse GitHub markdown, it don't provide a generic escape ?!? boo, hiss 😠 )
• added some ~ to a description with a lot of long module paths

[pp-mo]

Some ideas also occurred to me, to improve the Ancillary Data features entry...
I think this should highlight the new classes and cube access features, as well as describing the netcdf format handling.
(though it already diverts a bit to explain Flags).

But maybe I should just create another PR for that?
I am also wondering whether it would be worth making this into a 'featured change' or 'showcase feature' box,
something like https://scitools-iris.readthedocs.io/en/latest/whatsnew/1.12.html#features

? What do you think @trexfeathers @bjlittle

[trexfeathers]

I think this should highlight the new classes and cube access features, as well as describing the netcdf format handling.

@pp-mo I'm afraid, having not worked on this area of the code at all, I don't have an educated view on the correct amount of detail. This entry was the one merged from the feature branch, so I had assumed that all 3 of you working on it had seen it at the time.

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But maybe I should just create another PR for that?

@pp-mo that would be good Git practice I think

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I am also wondering whether it would be worth making this into a 'featured change' or 'showcase feature' box,

@pp-mo I was imagining that something of this sort should be written once we're sure nothing else is getting merged. I'm not sure how we decide what goes into it, since none of us have touched all the new features.

[pp-mo]

Sorry, a bunch of commits targetting the whatsnew were just merged, causing conflicts here, and it seemed simpler to rebase, so I did.
I think we've more-or-less done with the prior discussion, and it's pretty easy to just re-check the new commits.
If you object, I can try a merge-back approach instead.

[pp-mo]

But maybe I should just create another PR for that?

@pp-mo that would be good Git practice I think

See #3865