Clarify &mut-methods' docs on sync::OnceLock

[lukaslueg]

Three small changes to the docs of sync::OnceLock:

• The docs for OnceLock::take() used to say "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(), Cell::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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[mejrs]

I like how Mutex and RefCell phrase this: "since this method borrows the $thing mutably, ..". What about something similar like "since this method borrows the OnceLock mutably, no synchronization is necessary"?