Adding a doc comment talking about performance impacts of `assume`

[tesuji]

So LLVM hasn't changed much since nagisa wrote #54995 (comment).
Here the link to related regression caused by using constly assume intrinsic in const fn: #77023 (comment)

I think we could include a warning for assume intrinsic about using it in const fn and should run a perf for PR using it.

rust/library/core/src/intrinsics.rs

Lines 735 to 737 in ced813f

	/// This intrinsic does not have a stable counterpart.
	#[rustc_const_unstable(feature = "const_assume", issue = "76972")]
	pub fn assume(b: bool);

cc @oli-obk

[RalfJung]

Is there anything specific about const here? Seems like what @nagisa wrote applies equally to fn and const fn.

[lolbinarycat]

triage: there is now a note about optimizations, and assert_unchecked also has much more explanatory documentation