std::iter: document iteration over `&T` and `&mut T`

A colleague of mine is new to Rust, and mentioned that it was “slightly
confusing” to figure out what `&mut` does in iterating over `&mut foo`:

```rust
for value in &mut self.my_vec {
    // ...
}
```

My colleague had read the `std::iter` docs and not found the answer
there. There is a brief section at the top about “the three forms of
iteration”, which mentions `iter_mut`, but it doesn’t cover the purpose
of `&mut coll` for a collection `coll`. This patch adds an explanatory
section to the docs. I opted to create a new section so that it can
appear after the note that `impl<I: Iterator> IntoIterator for I`, and
it’s nice for the existing “three forms of iteration” to appear near the
top.

Implementation note: I haven’t linkified the references to `HashSet` and
`HashMap`, since those are in `std` and these docs are in `core`;
linkifying them gave an “unresolved link” rustdoc error.

Test Plan:
Ran `./x.py doc library/core`, and the result looked good. Manually
copy-pasted the two doctests into the playground and ran them.

wchargin-branch: doc-iter-by-reference
wchargin-source: 0f35369a8a735868621166608797744e97536792

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