bevy_ecs: Add doc example for par_iter_mut (#11311)

[lkolbly]

Objective

Fixes #11311

Solution

Adds an example to the documentation for par_iter_mut. I didn't add any examples to par_iter, because I couldn't think of a good example and I figure users can infer that par_iter and par_iter_mut are similar.

[alice-i-cecile]

Really solid, thanks :)

[hymm]

Would be nice to have this on Query::par_iter_mut, but that could be a follow up pr.

[alice-i-cecile]

Waiting until Monday: add the docs there too if you get a chance. Otherwise I'll make a follow-up issue.

[lkolbly]

Would be nice to have this on Query::par_iter_mut, but that could be a follow up pr.

Is that a different Query than the one in crates/bevy_ecs/src/system/query.rs?

[hymm]

Is that a different Query than the one in crates/bevy_ecs/src/system/query.rs?

Oops, not sure what I was thinking with that comment. Might have been to add them to par_iter instead and was thinking about something else at the same time.

[lkolbly]

Fair 'nuff. I can't think of any good examples for a succinct par_iter use case, they all involve atomics or locks or channels or some sort of synchronization. But, I added a note:

    /// Note that you must use the `for_each` method to iterate over the
    /// results, see [`par_iter_mut`] for an example.

which hopefully should point users in the right direction?