[LangRef] update according to unwinding support in inline asm

https://reviews.llvm.org/D95745 introduced a new `unwind` keyword for inline assembler expressions. Inline asms marked with the `unwind` keyword allows stack unwinding from inline assembly because the compiler emits unwinding information ("around" the inline asm) as it would for calls/invokes. Unwinding the stack from within non-unwind inline asm may cause UB.

Reviewed By: Amanieu

Differential Revision: https://reviews.llvm.org/D102642