Expand docs for on_link_click in Rich Text

[Zarthus]

0.14-dev introduces the requirement to set on_link_clicked for any rich text. The behaviour was not very intuitive to me, and I don't fully understand all the intricacies.

That said, I would not have come to the conclusion to call on_link_clicked(never) (primarily because it seems to hold a Some/None value, setting to a Some(Box::new(never)) did not feel intuitive to me) without asking somebody else for help, so documenting this permanently somewhere can't hurt.

There's probably a better improvement that can be done somehow. I did consider adding a helper method (no_link or something) that does what the documentation suggests, but I also didn't want to add extra potential bloat.