libstd/rustdoc: Document potential panics in RefCell where `.borrow()` is used implicitly in impls

[Centril]

There are a bunch of implicit uses of .borrow(), which may panic if the RefCell is already mutably borrowed, inside impls of RefCell:

• PartialEq
• Clone
• PartialOrd
• Ord

The potential panics should be noted with a header:

    /// # Panics
    ///
    /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
    /// [`try_borrow`](#method.try_borrow).

I'll file the PR fixing this if we agree that this is a problem in need of fixing.