Rollup merge of rust-lang#48365 - Centril:docs/document-refcell-panics, r=frewsxcv

RefCell: document panics in Clone, PartialEq, PartialOrd, Ord.

This fixes rust-lang#47400 by adding:

```rust
    /// # Panics
    ///
    /// Panics if the value is currently mutably borrowed.
```
to said impls. They may panic since they call `.borrow()`.

Author: kennytm

src/libcore/cell.rs

@@ -863,6 +863,9 @@ impl<T: ?Sized> !Sync for RefCell<T> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Clone> Clone for RefCell<T> {
+    /// # Panics
+    ///
+    /// Panics if the value is currently mutably borrowed.
     #[inline]
     fn clone(&self) -> RefCell<T> {
         RefCell::new(self.borrow().clone())
@@ -880,6 +883,9 @@ impl<T:Default> Default for RefCell<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
+    /// # Panics
+    ///
+    /// Panics if the value in either `RefCell` is currently borrowed.
     #[inline]
     fn eq(&self, other: &RefCell<T>) -> bool {
         *self.borrow() == *other.borrow()
@@ -891,26 +897,41 @@ impl<T: ?Sized + Eq> Eq for RefCell<T> {}
 
 #[stable(feature = "cell_ord", since = "1.10.0")]
 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
+    /// # Panics
+    ///
+    /// Panics if the value in either `RefCell` is currently borrowed.
     #[inline]
     fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
         self.borrow().partial_cmp(&*other.borrow())
     }
 
+    /// # Panics
+    ///
+    /// Panics if the value in either `RefCell` is currently borrowed.
     #[inline]
     fn lt(&self, other: &RefCell<T>) -> bool {
         *self.borrow() < *other.borrow()
     }
 
+    /// # Panics
+    ///
+    /// Panics if the value in either `RefCell` is currently borrowed.
     #[inline]
     fn le(&self, other: &RefCell<T>) -> bool {
         *self.borrow() <= *other.borrow()
     }
 
+    /// # Panics
+    ///
+    /// Panics if the value in either `RefCell` is currently borrowed.
     #[inline]
     fn gt(&self, other: &RefCell<T>) -> bool {
         *self.borrow() > *other.borrow()
     }
 
+    /// # Panics
+    ///
+    /// Panics if the value in either `RefCell` is currently borrowed.
     #[inline]
     fn ge(&self, other: &RefCell<T>) -> bool {
         *self.borrow() >= *other.borrow()
@@ -919,6 +940,9 @@ impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
 
 #[stable(feature = "cell_ord", since = "1.10.0")]
 impl<T: ?Sized + Ord> Ord for RefCell<T> {
+    /// # Panics
+    ///
+    /// Panics if the value in either `RefCell` is currently borrowed.
     #[inline]
     fn cmp(&self, other: &RefCell<T>) -> Ordering {
         self.borrow().cmp(&*other.borrow())