Updated CHANGELOG.md for post 1.5.1 release & minor cleanup

[rgoldberg]

Updated CHANGELOG.md for post 1.5.1 release & minor cleanup:

• Noted that longstanding completion issues still remain, and will be fixed over time.
• Cleaned up a few issues in the existing changelog
• Can provide highlights about the completion improvements if you want me to make a few more commits

Sorry for not properly noticing earlier that 1.5.1 would only include the minor patch, not all the other post 1.5.0 improvements currently in main.

Thanks again, and thanks for the shout out in the notes.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

LGTM!

[natecook1000]

@swift-ci Please test