[LangRef] Make lifetime intrinsic's semantics consistent with StackColoring's comment

This patch is an update to LangRef by describing lifetime intrinsics' behavior
by following the description of MIR's LIFETIME_START/LIFETIME_END markers
at StackColoring.cpp (https://github.com/llvm/llvm-project/blob/eb44682d671d66e422b02595a636050582a4d84a/llvm/lib/CodeGen/StackColoring.cpp#L163) and the discussion in llvm-dev.

In order to explicitly define the meaning of an object lifetime, I added 'Object Lifetime' subsection.

Reviewed By: nlopes

Differential Revision: https://reviews.llvm.org/D94002