fix incorrect docs for add_*_script
fix incorrect docs for add_*_script

They claimed that all of these functions accepted any posargs or varargs
that install scripts supported. This was inconsistent with both the
types that meson currently allowed, and the types that we documented in
refman 1.0 *should* be allowed.

Take the opportuninty to be clear as refman 1.0 never was, about the
difference between types supported in the first posarg, and the ypes
supported in succeeding varargs.

mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson