[LangRef] clarify the semantics of nocapture

This patch clarifies the semantics of nocapture attribute.

A 'Pointer Capture' subsection is added to describe the semantics of pointer capture first.

For the nocapture example with two same pointer arguments, it is consistent with the semantics that Alive2 used to run lit tests.

Reviewed By: nlopes

Differential Revision: https://reviews.llvm.org/D97924