The `iter_many_mut` documentation is confused

[nicopap]

Calls a closure on each result of Query where the entities match.

The documentation talks about calling a "closure", but iter_many_mut doesn't have a closure argument, as the example shows. https://docs.rs/bevy/latest/bevy/ecs/system/struct.Query.html#method.iter_many_mut

[Nilirad]

I'll rebase #4989 to main and add more appropriate doc comments.

[ian-h-chamberlain]

To add to this, it would be helpful to add a mention that the returned value is not actually an Iterator. I was confused by the errors when I tried

for foo in q.iter_many_mut(&entities) {}

The comment on the Iterator impl for QueryManyIter explains this a bit:

/// Iterator type is intentionally implemented only for read-only access.
/// Doing so for mutable references would be unsound,
/// because calling `next` multiple times would allow multiple owned references to the same data to exist.

but it might be good to mention as well on iter_many_mut as well, to explain the need for this usage (which can easily be misread as the more common while let Some(_) = it.next():

let mut it = q.iter_many_mut(&entities)
while let Some(foo) = it.fetch_next() {}