Remove claim of atomicity from Copy (#11505)

* Remove claim of atomicity from Copy

* minor

---------

Co-authored-by: Andy (Steve) De George <[email protected]>

Author: Smaug123

xml/System.IO/File.xml

@@ -3656,7 +3656,7 @@ Note that if you attempt to replace a file by moving a file of the same name int
 
 - In .NET Core 3.0 and later versions, you can call <xref:System.IO.File.Move(System.String,System.String,System.Boolean)> setting the parameter `overwrite` to `true`, which will replace the file if it exists.
 
-- In all .NET versions, you can call <xref:System.IO.File.Copy(System.String,System.String,System.Boolean)> to copy with overwrite, then call `Delete` to remove the excess source file. This strategy is advisable if the file being copied is small, and you are looking for an "atomic" file operation. If you `Delete` the file first, and the system or program crashes, the destination file will no longer exist.
+- In all .NET versions, you can call <xref:System.IO.File.Copy(System.String,System.String,System.Boolean)> to copy with overwrite, then call `Delete` to remove the excess source file. This strategy isn't atomic, because a system or program crash during the `Copy` may leave a partially-written destination file, but it will ensure a (possibly incomplete) file always exists at the destination.
 
 - In all .NET versions, you can call <xref:System.IO.File.Delete(System.String)> before calling `Move`, which will only delete the file if it exists.