free: --si changes base of default output

Previously, if '--si' was the only used option, it was ineffective, as it is (implicitly from manpage) supposed to be combined with '-h'. Now sole '--si' has the same effect as '--kilo', however since we already have that option, why not to make it do what it is supposed to, i.e. change the base of output.

The suggestion is to either merge this or extend '--si' explanation with '-h'-pairing to be effective in manpage.

Fedora Bugzilla https://bugzilla.redhat.com/show_bug.cgi?id=2083789