14676 wpf docs

[mrlacey]

PR checklist

Quick summary of changes

Update documentation references to previously deprecated "frameworks" that have since been removed.

I also tidied up some spelling and basic formatting issues while I had these docs open.

Which issue does this PR relate to?

#4676

Applies to the following platforms:

• [ ] WinUI
• [x] WPF
• [ ] UWP

Anything that requires particular review or attention?

no

Do all automated tests pass?

didn't check - it's a docs only change

Have automated tests been added for new features?

n/a

If you've changed the UI:

• Be sure you are including screenshots to show the changes.
• Be sure you have reviewed the accessibility checklist.

n/a

If you've included a new template:

• Be sure you reviewed the Template Verification Checklist.
• Be sure it's included in the list on this UWP or WPF getting started doc.

n/a

Have you raised issues for any needed follow-on work?

n/a

Have docs been updated?

YEP!

If breaking changes or different ways of doing things have been introduced, have they been communicated widely?

n/a