Rename `EventReader::iter`

[alice-i-cecile]

          Agreed. I am also in favor of renaming `.iter()` to something clearer like `.read()`. Having a function named `.iter()` that modifies state has always rubbed me the wrong way.

Originally posted by @JoJoJet in #9583 (review)

[alice-i-cecile]

I've seen quite a few beginners trip over this. They're generally confused by:

1. Why do we need a mutable reference to the event reader to read the events?
2. Does this mean we can only read events in one system?
3. If the events are double-buffered, does this mean we'll double-read events?

I think in all cases swapping to read will help clarify things.

[JoJoJet]

One objection I foresee is that calling it 'read' also implies a read-only operation. However, read being a stateful operation is idiomatic rust: see std::io::Read as one example.

[Alfiec7113]

I've seen quite a few beginners trip over this. They're generally confused by:

1. Why do we need a mutable reference to the event reader to read the events?
2. Does this mean we can only read events in one system?
3. If the events are double-buffered, does this mean we'll double-read events?

I think in all cases swapping to read will help clarify things.

I can confirm as someone new to bevy that all 3 of these points tripped me up

[EmiOnGit]

Issue has been resolved by #9631 and could therefore be closed