[doc]Small modification on the WeakHashMap doc

[Charles Lee]

Hi guys,

In the Implementation notes of WeakHashMap[1], says:

/One way to deal with this is to wrap values themselves within 
WeakReferences before inserting, as in: m.put(key, new 
WeakReference(value)), and then unwrapping upon each get./

However, it is not concise and a little misleading. Because the value in 
the WeakReference can be GC'd if there are no strong reference to it. 
This behaviour surprises some customers.
How about add a statement like [2]:

/However, as the use of WeakReference in this manner will not prevent 
value objects from being GC'd, this approach is only useful when entries 
in the map are not relied upon for keeping the underlying value objects 
"live"./




[1]: http://docs.oracle.com/javase/7/docs/api/java/util/WeakHashMap.html
[2]: http://cr.openjdk.java.net/~littlee/7166055/webrev.00/ 
<http://cr.openjdk.java.net/%7Elittlee/7166055/webrev.00/>

-- 
Yours Charles