Rollup merge of rust-lang#128457 - juntyr:once-lock-docs, r=tgross35

Fix docs for OnceLock::get_mut_or_init

Removes an incorrect statment about concurrency from the `OnceLock::get_mut_or_init` (tracked in rust-lang#121641) docs.

Fixes rust-lang#128429

Author: matthiaskrgr

library/std/src/sync/once_lock.rs

@@ -281,9 +281,7 @@ impl<T> OnceLock<T> {
     /// Gets the mutable reference of the contents of the cell, initializing
     /// it with `f` if the cell was empty.
     ///
-    /// Many threads may call `get_mut_or_init` concurrently with different
-    /// initializing functions, but it is guaranteed that only one function
-    /// will be executed.
+    /// This method never blocks.
     ///
     /// # Panics
     ///
@@ -373,6 +371,8 @@ impl<T> OnceLock<T> {
     /// it with `f` if the cell was empty. If the cell was empty and `f` failed,
     /// an error is returned.
     ///
+    /// This method never blocks.
+    ///
     /// # Panics
     ///
     /// If `f` panics, the panic is propagated to the caller, and