Update docs for FancyArrowPatch & Annotation to make it clear that ShrinkA/B parameters are in points and not fractional.

[thehappycheese]

PR summary

closes #27941

Updates documentation in two places to make it more clear that FancyArrowPatch uses point units for the shrinkA & shrinkB parameters.

This was particularly unclear when accessing FancyArrowPatch via Axes.annotate() because there is an alternate "simple" way to specify an arrow that uses shink as a fraction rather than as points.
Therefore I have also proposed changes to the Annotation class in the text module.

Also please summarize the changes in the title, for example "Raise ValueError on
non-numeric input to set_xlim" and avoid non-descriptive titles such as "Addresses
issue #8576".

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• [N/A] new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

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[thehappycheese]

It could be argued that the second commit/change to Annotation is not needed since the unit is specified in the default value.

However I personally failed to spot that because Annotation also accepts a simple arrow format where shrink is fractional. When switching from simple to fancy I could not spot the problem. Therefore I still included it in this PR.

I would be happy to re-submit this PR without the change to Annotation if the maintainers feel it isnt needed.

I am pretty sure the change to FancyArrowPatch is needed though.

[timhoffm]

Thanks @thehappycheese, and congratulations on your first PR merged into Matplotlib! We hope to hear from you again.