comp: character vector of length one or two indicating which columns out of "Variance" and "Std.Dev." should be shown in the formatted output.
formatter: the function to be used for formatting the standard deviations and or variances (but not the correlations which (currently) are always formatted as "0.nnn"
useScale: whether to report a scale parameter (e.g. residual standard deviation)
...: optional arguments for formatter(*) in addition to the first (numeric vector) and digits.
Returns
a character matrix of formatted VarCorr entries from varc.