Add missing documentation to bevy_time

[joseph-gio]

Objective

• Add documentation to undocumented items in bevy_time.
• Add a warning for undocumented items.

[alice-i-cecile]

Progress towards #3492

[Selene-Amanita]

Not introduced by this PR but can be fixed by it, wrong backticks in https://docs.rs/bevy/0.11.0/bevy/time/fixed_timestep/struct.FixedTime.html#method.expend:
[`Err(FixedUpdateError`)]
(should probably link to FixedUpdateError not Err too)

(the doc of expend is not clear at all either, which I guess is fine since it's not supposed to be used by a lot of people, only people who make tests or make their own runner, but maybe the doc should say that then?)

[Selene-Amanita]

The doc on this page is great: https://docs.rs/bevy_time/0.11.0/bevy_time/fixed_timestep/index.html but FixedTime and run_fixed_update_schedule don't link back to it. Since FixedUpdate links to run_fixed_update_schedule, I think it would be great to link back to the central doc on fixed time, people (me at least) don't always think to check module documentation.

[joseph-gio]

Thank you for the review. I believe I have addressed your points.

[Selene-Amanita]

Besides a small typo looks good to me!

[GuillaumeGomez]

To help you with missing docs, you can add #![deny(missing_docs)]. :)

[alice-i-cecile]

@GuillaumeGomez a warn equivalent was added in lib.rs :) Like we discussed in the linked issue, warn is less intrusive for development, but still blocks CI.

[GuillaumeGomez]

Nice! There is also rustdoc::missing_doc_code_examples which could be helpful but I think it's nightly only. Bevy docs is really lacking examples. ^^'