Tell the world how to convert gigabytes to bytes with units(1) easily

It turns out that units(1) is not as horrible to use in scripts
as I initially thought. When the --terse flag is combined
with an appropriate output format (set via --output-format),
units(1) is actually capable of producing very nice results.
For example:

units -o %0.f -t '4 gigabytes' bytes

is is just going to print out the expected value of 4294967296.

There is no time to waste. People have to know about it.

I am adding an example for this at the top of the examples section
because this is what users are most likely looking for.

Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D24096