Clarify US unit and origin of data for mpg documentation

This is a suggestion for a minimal change in the documentation of the mpg dataset, and I am happy to provide a PR.

Currently, ?mpg gives the following description:

This dataset contains a subset of the fuel economy data that the EPA makes available on https://fueleconomy.gov/. It contains only models which had a new release every year between 1999 and 2008 - this was used as a proxy for the popularity of the car.

And the description for fuel efficiency variables:

cty
city miles per gallon
hwy
highway miles per gallon

There is no mention of the data originating from the US anywhere on the page.

Specifically, I would like to make it more explicit that:

• the EPA is the US EPA (as there are many other agencies that share the name (and possibly, also spell out "EPA" entirely)
• the units are miles per US gallons, and not miles per imperial gallons. (To be safe, I verified the factor by using the Fuel Economy website's own unit conversion setting.)

Start with the ?mpg help page and locate the documentation for the dataset and its cty and hwy variables. Update it to identify the US EPA and state that fuel-efficiency values use miles per US gallons, then verify the rendered help page reflects both clarifications.