Document that null is 0

[joshlf]

This has come up in two places recently: #115333 (comment) and #116675 (comment).

We should do the following:

• On ptr::null(), document that ptr::null() is equivalent to 0usize as *const _
• Either before or after it lands, update the text in #116675 to document that "allocated objects" cannot contain the 0 address (alternatively, if that PR lands with text that says that an allocated object cannot contain the "null" address, update the text to mention that null is equal to 0)

Are there other places we should update too?

cc @the8472 @tmandry @RalfJung

[Nilstrieb]

there was some discussion in https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/Null.20vs.200

[RalfJung]

document that ptr::null() is equivalent to 0usize as *const _

Int-to-ptr casts are terrible, I'd rather not do that. In fact these two are not equivalent for Miri (but that might be a Miri limitation, int2ptr is just hard or impossible to handle precisely for a tool like Miri).

We should say that it returns a pointer whose address is 0, but we shouldn't commit to which provenance it has.

[RalfJung]

#116988 adds the docs for this.