Update help file for `coefplot()`

[davidaarmstrong]

I recently answered this question on StackOverflow and there was some confusion about the relationship between the x and keep arguments. It might be worth fleshing that out in the help file a bit. For example:

x - a vector of values the same length as the vector of coefficients providing alternative labels for the corresponding coefficient values. keep - Add to the existing description - If x is specified, these regular expressions will be executed on the labels provided in x.