doc/flux-content(1): update backing store description

Problem: flux-content(1) describes old behavior of
content backing store module at unload time, where content
was copied from the backing store to the memory cache.
This is no longer done.

Update the description to reflect the way it works now.

doc/man1/flux-content.adoc

@@ -68,10 +68,9 @@ cache of its availability, which triggers the cache to begin
 offloading entries.  Once entries are offloaded, they are eligible
 for expiration from the rank 0 cache.
 
-If the module is unloaded, its contents are transferred back
-to the cache.  This operation may fail if more content has
-been stored than can fit in memory, and so is only advisable early
-in the life of an instance.
+To avoid data loss, once a content backing module is loaded,
+do not unload it unless the content cache on rank 0 has been flushed
+and the system is shutting down.
 
 
 CACHE EXPIRATION