Fix setRouteLeaveHook references in docs

[nvartolomei]

Basically the text for API docs is copied from createTransitionManager, looks like it is good, though didn't had chance to dig into router's internals.
Removed last line Returns a function that may be used to unbind the listener., as react-router manages this itself.

Let me know what you think. Hope got it right.

Fixes #3488

[taion]

You can still unbind the listener yourself... though maybe it's not so necessary.

The comment is also not quite correct – you can return true or undefined (or actually any non-string value other than false) to not abort the transition.

[nvartolomei]

@taion just amended the commits, though some rewording may be still needed. Feel free to contribute 🤗

[taion]

Still not quite right – you can return

1. A string to show a prompt
2. false to cancel the transition
3. Anything else (or nothing) to let the transition continue

[nvartolomei]

During a normal transition, the hook function receives the next location as its only argument and can return a prompt message (string) to show the user, to make sure they want to leave the page, false, to prevent the transition. Any other return values will not have any effect.

[taion]

During a normal transition, the hook function receives the next location as its only argument and can return either a prompt message (string) to show the user, to make sure they want to leave the page; or false, to prevent the transition. Any other return value will have no effect.

Thanks!

[nvartolomei]

@taion amended once again. Please let me know if I should update the comment for listenBeforeLeavingRoute to match the one in docs.

[taion]

I wasn't going to make you do it, but now that you volunteered, please do. This is LGTM.