docs: add clarity about the 1000 events history

When talking about missing events on a particular key, the 1000 event history 
limit can be understood as being per key, instead of etcd-wide events. Make it 
clear that it is across all etcd keys.

Documentation/api.md

@@ -287,7 +287,7 @@ curl 'http://127.0.0.1:2379/v2/keys/foo?wait=true&waitIndex=7'
 
 The watch command returns immediately with the same response as previously.
 
-**Note**: etcd only keeps the responses of the most recent 1000 events. 
+**Note**: etcd only keeps the responses of the most recent 1000 events across all etcd keys. 
 It is recommended to send the response to another thread to process immediately
 instead of blocking the watch while processing the result.