[LangRef] Improve wording DW_OP_LLVM_entry_value

This commit simplifies the text of DW_OP_LLVM_entry_value by making it
terser, replacing a verbose example with a more concrete one, providing
an explicit conclusion on the meaning of N=1, and by transforming the
description of which passes generate this op into a list (which enables
future expansion of this list).

Differential Revision: https://reviews.llvm.org/D149177