[docs] Fix self-contradictory description of llvm_unreachable

[docs] Fix self-contradictory description of llvm_unreachable

Just two paragraphs above it says:
"If the compiler does not support this [skipping code generation for a particular branch], it will fall back
to the "abort" implementation."

And that actually correctly describes llvm_unreachable implementation.

Differential Revision: https://reviews.llvm.org/D81130