docs: buffer and ReplaySubject breaking changes (#6268)

* docs: add buffer breaking change

* docs: add ReplaySubject breaking change

docs_app/content/6-to-7-change-summary.md

@@ -177,6 +177,10 @@ This document contains a detailed list of changes between RxJS 6.x and RxJS 7.x,
 
 - `ReadableStream` such as those returned by `fetch`, et al, can be passed to any API that accepts an observable, and can be converted to `Observable` directly using `from`.
 
+### ReplaySubject
+
+- A [bug was fixed](https://github.com/ReactiveX/rxjs/pull/5696) that prevented a completed or errored `ReplaySubject` from accumulating values in its buffer when resubscribed to another source. This breaks some uses - like [this StackOverflow answer](https://stackoverflow.com/a/54957061) - that depended upon the buggy behavior.
+
 ### Subscription
 
 - Now allows adding and removing of functions directly via `add` and `remove` methods.
@@ -189,6 +193,10 @@ This document contains a detailed list of changes between RxJS 6.x and RxJS 7.x,
 
 ## Breaking Changes
 
+### buffer
+
+- `buffer` now subscribes to the source observable before it subscribes to the closing notifier. Previously, it subscribed to the closing notifier first.
+
 ### combineLatest
 
 - Generic signatures have changed. Do not explicitly pass generics.