improve documentation of RefCell::get_mut

[comex]

From IRC:

<bletch>	how do I set the value of a RefCell? all the examples I see are about mutating it, not setting the value
<Mutabah>	You can overwrite the existing value if you can mutate
<Mutabah>	Just by doing `*cell.borrow_mut() = new_value`
<bletch>	but then it says it's an immutable borrowed content
<bletch>	when inside TLS
<bletch>	"cannot borrow immutable borrowed content as mutable"
<bletch>	with(|f| *f.get_mut() = new_gc)
[..]
<bletch>	ok, borrow_mut fixed it
<bletch>	yeah, I was using get_mut
<bletch>	I don't know why they're different

The documentation for get_mut is not terrible:

Returns a mutable reference to the underlying data.
This call borrows RefCell mutably (at compile-time) so there is no need for dynamic checks.

but I think it could be more explicit that get_mut is for special circumstances / usually not what you want. The name get_mut may tend to confuse newbies since it looks 'basic' and very similar to borrow_mut.

[steveklabnik]

doc team triage: p-medium

[GuillaumeGomez]

Took a look at it and the prototype of the method seems clear enough to me:

fn get_mut(&mut self) -> &mut T

It clearly expects self to be mutable. However, I'll add something about it to be clear but I don't think this is really necessary...