Clarify --packaging option

[vt-alt]

rinstall support the packagers use-case out of the box. When calling rinstall inside a package specification (i.e. spec file, PKGBUILD, ebuild), add the --packaging flag and it will enable all relevant flags and ask you the needed information:

Can you clarify the change of behavior? What are "relevant flags"? And generally we don't need any "asking" when packaging.

From #4 we see

--packaging would enable --system, requires --destdir and disable the installation of pkg.info files.

So perhaps this should be stated instead of reassuring words.

Also, when packaging we never ever want to install into local system prefix /usr/local we want to install into a vendor prefix /usr, so there is incomplete packaging support out of the box.