Document unaligned reference restrictions

[ehuss]

cc rust-lang/rust#102513

[dtolnay]

As part of this, please also delete this outdated warning:

reference/src/type-layout.md

Lines 552 to 558 in b9ccb09

	<div class="warning">
	***Warning:*** Dereferencing an unaligned pointer is [undefined behavior] and
	it is possible to [safely create unaligned pointers to `packed` fields][27060].
	Like all ways to create undefined behavior in safe Rust, this is a bug.
	</div>

reference/src/type-layout.md

Line 590 in b9ccb09

[27060]: https://github.com/rust-lang/rust/issues/27060

[ehuss]

#1367 has removed the warning, but there still needs to be something added about the restriction of creating unaligned references (probably in the pointers page?). Also, I don't think "unaligned" is ever explicitly defined, which probably should be covered.