Guide does not explain the namespaced enums

[lifthrasiir]

We have lots of the legacy of non-namespaced enums throughout the guide:

• Section 8.4 and 9 should use Ordering::{Less, Equal, Greater} instead of {Less, Equal, Greater} (cf. The enum type in Guide is not working #17967, Update guide to reflect changes to enums. #19551).
• Section 8.4 contains StringResult::{StringOK, ErrorReason}, which probably should be renamed.
• Section 22 contains OptionalFloat64::{Valuef64, Missingf64}, which probably should be renamed.
• Section 22 introduces a generic Option<T>, but doesn't mention that its variants are reexported by default. (In fact, the guide doesn't mention that variants can be reexported at all.)

[steveklabnik]

• It's in the prelude, so it's fine.
• These are named this way because we haven't introduced Generics yet
• Same here
• Same deal, prelude.

That being said, mentioning that enums are namespaced would be a good idea, so I'm going to leave this ticket open as a TODO to add that.

[steveklabnik]

(I actually thought I already had such a TODO issue, but cannot find it.)