Rollup merge of rust-lang#87685 - notriddle:lazy-from-docs, r=dtolnay

Write docs for SyncOnceCell From and Default impl

Part of rust-lang#51430

library/std/src/lazy.rs

@@ -87,6 +87,19 @@ impl<T: UnwindSafe> UnwindSafe for SyncOnceCell<T> {}
 
 #[unstable(feature = "once_cell", issue = "74465")]
 impl<T> Default for SyncOnceCell<T> {
+    /// Creates a new empty cell.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// #![feature(once_cell)]
+    ///
+    /// use std::lazy::SyncOnceCell;
+    ///
+    /// fn main() {
+    ///     assert_eq!(SyncOnceCell::<()>::new(), SyncOnceCell::default());
+    /// }
+    /// ```
     fn default() -> SyncOnceCell<T> {
         SyncOnceCell::new()
     }
@@ -118,6 +131,23 @@ impl<T: Clone> Clone for SyncOnceCell<T> {
 
 #[unstable(feature = "once_cell", issue = "74465")]
 impl<T> From<T> for SyncOnceCell<T> {
+    /// Create a new cell with its contents set to `value`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// #![feature(once_cell)]
+    ///
+    /// use std::lazy::SyncOnceCell;
+    ///
+    /// # fn main() -> Result<(), i32> {
+    /// let a = SyncOnceCell::from(3);
+    /// let b = SyncOnceCell::new();
+    /// b.set(3)?;
+    /// assert_eq!(a, b);
+    /// Ok(())
+    /// # }
+    /// ```
     fn from(value: T) -> Self {
         let cell = Self::new();
         match cell.set(value) {