Rollup merge of #45052 - steveklabnik:gh44105, r=dtolnay

Rollup merge of #45052 - steveklabnik:gh44105, r=dtolnay

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