Improve documentation osrmIsochrone(): breaks argument

[JosiahParry]

The breaks arugment of osrmIsocrhone() says "a numeric vector of isochrone values (in minutes)." This language requires a bit of specialized knowledge. A google search for "isochrome values" does not resolve any confusions. My suspicion is that the breaks are intervals in the travel distance in units of minutes. But this is just a hunch.

Please document breaks a bit more clearly either in the argument itself or in the description. Per #106 I do not see a definition of "isochrone" in osrmIsochrone(), the readme, or any other functions man docs.