Clarify that SyncOnceCell::set blocks.

Reading the discussion of this feature, I gained the mistaken impression that neither `set` nor `get` blocked, and thus calling `get` immediately after `set` was not guaranteed to succeed. It turns out that `set` *does* block, guaranteeing that the cell contains a value once `set` returns. This change updates the documentation to state that explicitly.

library/std/src/lazy.rs

@@ -177,7 +177,10 @@ pub fn get_mut(&mut self) -> Option<&mut T> {
 
     /// Sets the contents of this cell to `value`.
     ///
-    /// Returns `Ok(())` if the cell's value was updated.
+    /// May block if another thread is currently attempting to initialize the cell. The cell is
+    /// guaranteed to contain a value when set returns, though not necessarily the one provided.
+    ///
+    /// Returns `Ok(())` if the cell's value was set by this call.
     ///
     /// # Examples
     ///