Move "gnop -s" example from md(8) to gnop(8).

[trasz]

Summary

Move "gnop -s" example from md(8) to gnop(8); it fits better there.
Also tweak the description a little.

[wblock]

The question addressed by that example was "how do I mount this disk image that has a bunch of header information that should be skipped over?"

And gnop(8) did not even mention that capability.

I'd suggest leaving the example in mdconfig.8 and expanding gnop.8 to describe more of its capabilities.

[trasz]

wblock@ is right; the point of this example is to point people to gnop, which they might not know about, so putting it inside gnop(8) would defeat its purpose.