Re-word rails#49856 CHANGELOG entry

Follow-up to [rails#49856][]

Replace shell word expansion with full English. In addition to that
change, elaborate on the changes with more detailed explanation.

[rails#49856]: rails#49856 (comment)

Author: seanpdoyle

actionview/CHANGELOG.md

@@ -11,7 +11,9 @@
 
     *Sean Doyle*
 
-*   Rename `ActionView::TestCase::Behavior::{Content,RenderedViewContent}`
+*   Rename `ActionView::TestCase::Behavior::Content` to `ActionView::TestCase::Behavior::RenderedViewContent`
+
+    Make `RenderedViewContent` inherit from `String`. Make private API with `:nodoc:`
 
     *Sean Doyle*