Rollup merge of #37652 - SimonSapin:arc-count-doc, r=alexcrichton

More proeminent warning in Arc::{strong,weak}_count docs.

CC #28356 (comment)

Author: steveklabnik

src/liballoc/arc.rs

@@ -319,11 +319,14 @@ impl<T: ?Sized> Arc<T> {
 
     /// Gets the number of [`Weak`][weak] pointers to this value.
     ///
-    /// Be careful how you use this information, because another thread
-    /// may change the weak count at any time.
-    ///
     /// [weak]: struct.Weak.html
     ///
+    /// # Safety
+    ///
+    /// This method by itself is safe, but using it correctly requires extra care.
+    /// Another thread can change the weak count at any time,
+    /// including potentially between calling this method and acting on the result.
+    ///
     /// # Examples
     ///
     /// ```
@@ -347,8 +350,11 @@ impl<T: ?Sized> Arc<T> {
 
     /// Gets the number of strong (`Arc`) pointers to this value.
     ///
-    /// Be careful how you use this information, because another thread
-    /// may change the strong count at any time.
+    /// # Safety
+    ///
+    /// This method by itself is safe, but using it correctly requires extra care.
+    /// Another thread can change the strong count at any time,
+    /// including potentially between calling this method and acting on the result.
     ///
     /// # Examples
     ///