Help: Fix minor inaccuracies of what BUILD_RPATH_USE_ORIGIN affects

Discussions in !2456 (comment 467710) raised the question of whether BUILD_RPATH_USE_ORIGIN could apply to macOS as well, but the MACOSX_RPATH target property already effectively implements equivalent functionality for Apple platforms. This MR adds a cross-reference to that and also cleans up some slight inaccuracies in the same docs, specifically that it applies to shared libraries too, not just executables.