Clarify relationship between individual modules and Quantity module?

[ianmackenzie]

It's not always obvious that you can use functions in the Quantity module to work with Length, Duration etc. values; could be useful to add something like this to the top of each module's documentation:

Note that a Duration is a Quantity Float Seconds, so most functions in the Quantity module (such as plus, greaterThan, and interpolateFrom) will work on Duration values.