Clarify kernel-param value as string

[aelsabbahy]

Clarify the value is a string, to avoid users doing something like this.

KernelParam: net.ipv4.ip_forward: value:
Expected
    <string>: 0
to equal
    <int>: 0

[walidshaari]

encourge people to do : goss add to find the syntax too, e.g.
"goss add kernel-param net.ipv4.ip_forward"

[zylad]

The PR lookss good. Any chance to get it merged?

[aelsabbahy]

Need to just update the docs for now.