Mention that asm! is unsafe

[N1ark]

this is my first PR to the reference, apologies if this should have gone through an issue or more discussion before opening

Add a line in unsafety.md and inline-assembly.md pointing out that asm! requires unsafe.

This is already briefly mentioned in inline-assembly.md, but feels very off-handed and lacklustre:

Programmers should exercise appropriate care, as invoking this unsafe capability comes with assuming the responsibility of not violating rules of both the compiler or the architecture.

Unrelatedly; i believe the header level for asm.validity is wrong and should only be an h2? since it currently sits inside "Rules for naked inline assembly", while mentioning asm!. note that it is immediately followed by "Directives support", which also seems to need to be a h2 rather than h3. happy to do those changes here or separately.

disclaimer i have very little knowledge on asm! -- i primarily care about the documentation surrounding unsafe to be exhaustive.