Add changelog entry for .reflections API change

`.reflections` public API changed to return a String instead of a Symbol
as keys.

see commit 1f31488499111fdfce79d8dc1cc8fb008f7cdb25 and 6259e4e2dcca9a79f22f96658c33efe81936bc0d

[fixes #16928]
[fixes #17610]
Update reflections public API doc
Merge pull request #17718 from arthurnn/fixes_16928

Fixes 16928, improve reflections public API doc [ci skip]
Merge pull request #17719 from yui-knk/fix/rails_on_rack_guide

Fix explanation for ActionDispatch::Callbacks. [ci skip]
[ci skip] Dispatching is a verb :bow:

cc #17719
[ci skip] Fix class name of middleware.