Documentation: RPATH-related docs don't say it's actually setting RUNPATH

Near as I can tell from my research, RUNPATH is to be preferred over RPATH on platforms where RUNPATH is supported, so it's great that CMake appears to be setting RUNPATH whenever I use its RPATH functionality. But I only discovered that discussion after extensive searching for an answer as to why CMake didn't appear to have RUNPATH support. All of the RPATH related docs should mention clearly that cmake will set RUNPATH instead where it's available (or whatever the actual semantics are).

All of the conceptual information @ben.boeckel posted here should be in the same docs, too.