Get rid of `ok().expect()` in the docs

[fhartwig]

This discussion showed that some people are confused by the use of the .ok().expect() on Result values in the documentation. Since Result::expect is now stable, this pattern has become mostly obsolete (with the exception of Result types whose error types do not implement Debug, which shouldn't be very common).

It probably shouldn't be used in the docs any more. Probably the most important place where this should be fixed is https://github.com/rust-lang/rust/blob/6d43fef3aaf64b2e7df14ca676a3a39b723ed7f3/src/doc/trpl/guessing-game.md , which will need to be re-written a bit since the text explicitly references that pattern.

[frewsxcv]

Now that #29507 has merged, can this be closed?

[steveklabnik]

That PR only says it address part of the issue.

[fhartwig]

#29507 fixed all the instances of this pattern that I could find in the docs, except for the guessing game. The guessing game has a paragraph that talks about the ok().expect() thing, and I wasn't sure if we just want to delete that or rewrite it (and how to rewrite it).