doc: mark str representation example as "don't actually do this"

[birkenfeld]

The example here led to some confusion how to get a slice from a Vec<u8>: https://users.rust-lang.org/t/checking-for-incoming-data-on-tcpstream/5814/7

The example code should probably be flagged as "don't do this in practice, this is just to show the innards".

[steveklabnik]

I am willing to mentor anyone who wants to fix this

[JessRudder]

@steveklabnik I'm looking to do my first open source commit and this seems handleable. :) Should I follow the instructions for documentation in the contributing.md and see how far I can get on this?

[steveklabnik]

@JessRudder yup! If you need any specific help, leave a comment or drop by IRC :)

[kyrias]

(This should be closed now.)