E0793: Clarify that it applies to unions as well

[He1pa]

pick up inactive PR: #131472

Also:
Adjust the language slightly to be more consistent with other similar messages (was created instead of got created).
Add a short section on union.
Add an example line showing referencing a field in a packed struct is safe if the field's type isn't more strictly aligned than the pack.

r? compiler-errors

[rustbot]

Failed to set assignee to [compiler-errors]: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

[rustbot]

compiler-errors is not on the review rotation at the moment.
They may take a while to respond.

[Kivooeo]

Some nits, and can you squash commits please

[Kivooeo]

Suggested change

	aligned. `addr_of!` and raw pointers should be used the same way as is done
	for `struct` fields.
	aligned. Use `addr_of!` and raw pointers in the same way as for struct fields.

[He1pa]

Which version should I use?
Use addr_of! and raw pointers in the same way as for struct fields.
or
addr_of! and raw pointers should be used in the same way as for struct fields.

[Kivooeo]

I'd stick with first one because it is more direct and imperative

[xizheyin]

Some suggestions. :)

[xizheyin]

A reference was created to an unaligned field of a [packed] `struct` or `union`.

[xizheyin]

We could also add some futher explanation, for example,

The `#[repr(packed)]` attribute removes padding between fields, which can
cause fields to be stored at unaligned memory addresses. Creating references
to such fields violates Rust's memory safety guarantees and can lead to
undefined behavior in optimized code.