Modify Rc/Arc language around mutability

There are a few exceptions to the rule that Arc/Rc are immutable. Rather
than dig into the details, add "generally" to hint at this difference,
as it's kind of a distraction at this point in the docs.

Additionally, Arc's docs were slightly different here generally, so add
in both the existing language and the exception.

Fixes #44105

1 files changed, 4 insertions, 2 deletions

diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index 3b7dbd813cf..9481cd4e1a4 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -52,8 +52,10 @@ const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 /// also destroyed.
 ///
 /// Shared references in Rust disallow mutation by default, and `Arc` is no
-/// exception. If you need to mutate through an `Arc`, use [`Mutex`][mutex],
-/// [`RwLock`][rwlock], or one of the [`Atomic`][atomic] types.
+/// exception: you cannot generally obtain a mutable reference to something
+/// inside an `Arc`. If you need to mutate through an `Arc`, use
+/// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
+/// types.
 ///
 /// ## Thread Safety
 ///