Add BFCache section to multi reader writer proposal

Adds a section to the proposal to explain interaction with the
BFCache.

proposals/MultipleReadersWriters.md

@@ -222,9 +222,9 @@ await parentHandle.remove(); // successful
 ```
 
 ### Interactions with BFCache
-A page should not be made aware that it or another page is or has been in the BFCache. A fully active page could be made aware of a BFCached page if there is contention between locks they hold. The File System Access API operations/primitives hold locks that may cause this issue.
+The File System Access API operations/primitives hold locks in the BFCache. A page should not be made aware that it or another page is or has been in the BFCache. A fully active page could be made aware of a BFCached page if there is contention between locks they hold.
 
-To keep BFCache enabled when a site uses the File System Access API, a BFCached page should be evicted only on locking contention with a fully active page. This allows the site to have the performance gains of BFCache up until it would be made aware of the BFCache.
+To keep BFCache enabled when a site uses the File System Access API, a BFCached page must be evicted on locking contention with a fully active page. Otherwise, a file system lock held by a page will not affect the page's eligibility for BFCache. This allows the site to have the performance gains of BFCache up until it would be made aware of the BFCache.
 
 ## Alternatives Considered