Update forcemerge.asciidoc

Author: A-Hall

docs/reference/indices/forcemerge.asciidoc

@@ -10,8 +10,8 @@ This call will block until the merge is complete. If the http connection is
 lost, the request will continue in the background, and any new requests will
 block until the previous force merge is complete.
 
-WARNING: Force merge should only be called against *read-only indices*. Running 
-force merge against a read-write index can cause very large segments to be produced 
+WARNING: Force merge should only be called against indices that will *never be written to again*. Running 
+force merge against an activly written to index can cause very large segments to be produced 
 (>5Gb per segment), and the merge policy will never consider it for merging again until 
 it mostly consists of deleted docs. This can cause very large segments to remain in the shards.