Clarify LazyInitialize.EnsureInitialized (#3543)

Fixes #3541

Author: heaths

xml/System.Threading/LazyInitializer.xml

@@ -125,7 +125,7 @@ ExpensiveData dataToUse = LazyInitializer.EnsureInitialized(ref _data, ref _data
   
  This method may be used concurrently by multiple threads to initialize `target`.  
   
- In the event that multiple threads access this method concurrently, multiple instances of `T` may be created, but only one will be stored into `target`. In such an occurrence, this method will not dispose of the objects that were not stored. If such objects must be disposed, it is up to the caller to determine if an object was not used and to then dispose of the object appropriately.  
+ In the event that multiple threads access this method concurrently, multiple instances of `T` may be created, but only one will be stored into `target` and returned. In such an occurrence, this method will not dispose of the objects that were not stored. If such objects must be disposed, use an overload that takes a `valueFactory` and dispose of the object if it does not reference the same stored object.
   
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