Document "install/strip" special target for Ninja / Makefile generators
I am trying to fix https://bugreports.qt.io/browse/QTCREATORBUG-22047 in Qt Creatory by adding "install/strip" as a special target.
This would help with deploying of binaries on devices, which do not need to be unstripped.
My commit is being rejected because it's not a documented CMake feature.