[LangRef] mention that the lifetime intrinsics' description in LangRef isn't everything

This is a minor patch that addresses concerns about lifetime in D94002.

We need to mention that what's written in LangRef isn't everything about lifetime.start/end
and its semantics depends on the stack coloring algorithm's pattern matching of a stack pointer.

If the stack coloring algorithm cannot conclude that a pointer is a stack-allocated object, the pointer is conservatively
considered as a non-stack one because stack coloring won't take this lifetime into account while assigning addresses.

A reference from alloca to lifetime.start/end is added as well.

Differential Revision: https://reviews.llvm.org/D98112