add note about `AlreadyExists` to `create_new`

[anatawa12]

Fixes #119244

[rustbot]

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[workingjubilee]

Thanks!

@bors r+

[bors]

:pushpin: Commit 81f5175868eefa9a12b27377c9fc474d7033cee6 has been approved by workingjubilee

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit 81f5175868eefa9a12b27377c9fc474d7033cee6 with merge 1fef1524f2717afb76832f1620ae54201f96523e...