Clean up wording to reduce confusion

The description of the timeout parameter was worded misleadingly; it implied that the API would wait until the cluster reached the desired level and then stayed at that level for the timeout. I've tweaked the sentence to remove the risk of confusion.

docs/reference/cluster/health.asciidoc

@@ -38,9 +38,9 @@ status is controlled by the worst index status.
 
 One of the main benefits of the API is the ability to wait until the
 cluster reaches a certain high water-mark health level. For example, the
-following will wait till the cluster reaches the `yellow` level for 50
-seconds (if it reaches the `green` or `yellow` status beforehand, it
-will return):
+following will wait for 50 seconds for the cluster to reach the `yellow`
+level (if it reaches the `green` or `yellow` status before 50 seconds elapse,
+it will return at that point):
 
 [source,js]
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