Adjust documentation of `Arc::make_mut`

Related discussion in the users forum:
https://users.rust-lang.org/t/what-s-this-alleged-difference-between-arc-make-mut-and-rc-make-mut/63747?u=steffahn

Also includes small formatting improvement in the documentation of `Rc::make_mut`.

This commit makes the two documentations in question complete analogs. The previously claimed point in which
one "differs from the behavior of" the other turns out to be incorrect, AFAIK.

One remaining inaccuracy: `Weak` pointers aren't disassociated from the allocation but only from the contained
value, i.e. in case of outstanding `Weak` pointers there still is a new allocation created, just the
call to `.clone()` is avoided, instead the value is moved from one allocation to the other.

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