Documented the Apps and AppConfig APIs.
Updated a few doc paragraphs following the app-loading refactor.
Typo fix for exception message.
Merge pull request #2108 from jgastal/master

Typo fix for exception message.
Added a missing import and removed an unneeded import
Added release notes for app loading changes.
Copy-edited previous commit.

Thanks Tim for the review.
Fixed #21669 -- Typo in docs/ref/forms/fields.txt.

Thanks alex_koval for the report.