Update to Rc and Arc documentation to favor the Rc::clone(&ptr) syntax.

This is a followup of the discussion in rust-lang/rfcs#1954.

The solution chosen by the core team to address the problem tackled by the [the RFC](rust-lang/rfcs#1954) was to make the function call syntax Rc::clone(&foo) the idiomatic way to clone a reference counted pointer (over the method call syntax foo.clone()).
This change updates the documentation of Rc, Arc and their respective Weak pointers to reflect this decision and bring more exposure to the existence of the function call syntax.

Update to Rc and Arc documentation to favor the Rc::clone(&ptr) syntax.

This is a followup of the discussion in rust-lang/rfcs#1954.

The solution chosen by the core team to address the problem tackled by the [the RFC](rust-lang/rfcs#1954) was to make the function call syntax Rc::clone(&foo) the idiomatic way to clone a reference counted pointer (over the method call syntax foo.clone()).
This change updates the documentation of Rc, Arc and their respective Weak pointers to reflect this decision and bring more exposure to the existence of the function call syntax.

Rollup of 10 pull requests

- Successful merges: #42103, #42137, #42162, #42167, #42175, #42207, #42217, #42246, #42249, #42251
- Failed merges:

Update to Rc and Arc documentation to favor the Rc::clone(&ptr) syntax.

This is a followup of the discussion in rust-lang/rfcs#1954.

The solution chosen by the core team to address the problem tackled by the [the RFC](rust-lang/rfcs#1954) was to make the function call syntax Rc::clone(&foo) the idiomatic way to clone a reference counted pointer (over the method call syntax foo.clone()).
This change updates the documentation of Rc, Arc and their respective Weak pointers to reflect this decision and bring more exposure to the existence of the function call syntax.

Remove recommendation about idiomatic syntax for Arc::clone

I believe we should not make this recommendation. I don't want to argue that `Arc::clone` is less idiomatic than `arc.clone`, but that the choice is not clear cut and that we should not be making this kind of call in the docs.

The `.clone()` form has advantages too: it is more succinct, it is more likely to be understood by beginners, and it is more uniform with other `clone` calls, indeed with most other method calls.

Whichever approach is better, I think that this discussion belongs in a style guide or textbook, rather than the library docs. We don't talk much about idiomatic code in the docs, this place is pretty exceptional.

The recommendation is also not followed in this repo. It is hard to figure out how many calls there are of the `.clone()` form, but there are 1550 uses of `Arc` and only 65 uses of `Arc::clone`. The recommendation has existed for over two years.

The recommendation was added in rust-lang#42137, as a result of rust-lang/rfcs#1954. However, note that that RFC was closed because it was not necessary to change the docs (the original RFC proposed a new function instead). So I don't think an RFC is necessary here (and I'm not trying to re-litigate the discussion on that RFC (which favoured `Arc::clone` as idiomatic) in any case).

cc @nical (who added the docs in the first place; sorry :-) )

r? @alexcrichton (or someone else on @rust-lang/libs )