change log and doc fixes

[kdeldycke]

A couple of missing references and inconsistencies I spotted after 8.4.0 release:

• Missing PR references in changelog entries:
  • apply Unix line endings for shell completion output for windows compat #3466
  • Add capture mode to CliRunner and revert default fileno exposure #3391
• Imprecise wording in colliding options' selection
• Missing new API reference in Sphinx docs
• Detail about slot arbitration
• Missing .. versionadded:: 8.4.0 admonition in docstrings

This is a WIP PR that I will use along the stable branch development cycle and is to be merged before its release.

[davidism]

I'm ok with merging this now, assuming you think you got everything? Would like to release 8.4.1 today or tomorrow.

[kdeldycke]

I'm ok with merging this now, assuming you think you got everything? Would like to release 8.4.1 today or tomorrow.

Can you wait at least tommorrow? I am backlogging all your changes and notifications from the past 5 days! 😅

[davidism]

Sure. It's also ok if the docs get updated later, or if we need to make 8.4.2 with more fixes next. I'd just like to get the parameter source issue fixed for Flask.

[kdeldycke]

Sure. It's also ok if the docs get updated later, or if we need to make 8.4.2 with more fixes next. I'd just like to get the parameter source issue fixed for Flask.

Ah yes. Then go ahead with a 8.4.1 release. I'll catch up in 8.4.2.

[kdeldycke]

Thanks for the merge @davidism ! :)