asm!: clarify that nomem / readonly can access private memory

[lukas-code]

As pointed out by @Freax13 in #1350 (comment), we already allow asm! blocks with options(nomem) or options(readonly) to read from or write to private memory, which I believe refers to memory that is allocated from within the asm! block and not accessible from the outside (and please correct me if I'm wrong on this):

reference/src/inline-assembly.md

Lines 577 to 582 in c118ff6

	r[asm.rules.mem-same-as-ffi]
	- The set of memory locations that assembly code is allowed to read and write are the same as those allowed for an FFI function.
	- Refer to the unsafe code guidelines for the exact rules.
	- If the `readonly` option is set, then only memory reads are allowed.
	- If the `nomem` option is set then no reads or writes to memory are allowed.
	- These rules do not apply to memory which is private to the asm code, such as stack space allocated within the asm block.

Since we already allow this, this PR also documents it on the nomem and readonly options directly, in addition to the documentation that can be found lower on the same page. No functional changes intended.

closes #1350

r? @Amanieu

@rustbot label A-asm