Clarify &mut-methods' docs on sync::OnceLock
Three small changes to the docs of `sync::OnceLock`:
* The docs for `OnceLock::take()` used to [say](https://doc.rust-lang.org/std/sync/struct.OnceLock.html#method.take) "**Safety** is guaranteed by requiring a mutable reference." (emphasis mine). While technically correct, imho its not necessary to even mention safety - as opposed to unsafety - here: Safety never comes up wrt `OnceLock`, as there is (currently) no way to interact with a `OnceLock` in an unsafe way; there are no unsafe methods on `OnceLock`, so there is "safety" guarantee required anywhere. What we simply meant to say is "**Synchronization** is guaranteed...".
* I've add that phrase to the other methods of `OnceLock` which take a `&mut self`, to highlight the fact that having a `&mut OnceLock` guarantees that synchronization with other threads is not required. This is the same as with [`Mutex::get_mut()`](https://doc.rust-lang.org/std/sync/struct.Mutex.html#method.get_mut), [`Cell::get_mut()`](https://doc.rust-lang.org/std/cell/struct.Cell.html#method.get_mut), and others.
* In that spirit, the half-sentence "or being initialized" was removed from `get_mut()`, as there is no way that the `OnceLock` is being initialized while we are holding `&mut` to it. Probably a copy&paste from `.get()`
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