Update {rc, sync}::Weak::ptr_eq doc about comparing Weak::new

src/liballoc/rc.rs

@@ -1832,8 +1832,9 @@ impl<T: ?Sized> Weak<T> {
         }
     }
 
-    /// Returns `true` if the two `Weak`s point to the same value (not just values
+    /// Returns `true` if the two `Weak`s point to the same value (not just
-    /// that compare as equal).
+    /// values that compare as equal), or if both don't point to any value
+    /// (because they were created with `Weak::new()`).
     ///
     /// # Notes
     ///

src/liballoc/sync.rs

@@ -1550,15 +1550,15 @@ impl<T: ?Sized> Weak<T> {
         }
     }
 
-    /// Returns `true` if the two `Weak`s point to the same value (not just values
+    /// Returns `true` if the two `Weak`s point to the same value (not just
-    /// that compare as equal).
+    /// values that compare as equal), or if both don't point to any value
+    /// (because they were created with `Weak::new()`).
     ///
     /// # Notes
     ///
     /// Since this compares pointers it means that `Weak::new()` will equal each
     /// other, even though they don't point to any value.
     ///
-    ///
     /// # Examples
     ///
     /// ```