improve splitobs docstring (#187)

src/splitobs.jl

@@ -42,9 +42,13 @@ end
 
 Partition the `data` into two or more subsets.
 
-When `at` is a number (between 0 and 1) this specifies the proportion in the first subset.
-When `at` is a tuple, each entry specifies the proportion an a subset,
-with the last having `1-sum(at)`. In all there are `length(at)+1` subsets returned.
+When `at` is a number between 0 and 1, this specifies the proportion in the first subset.
+
+When `at` is an integer, it specifies the number of observations in the first subset.
+
+When `at` is a tuple, entries specifies the number or proportion in each subset, except
+for the last which will contain the remaning observations. 
+The number of returned subsets is `length(at)+1`.
 
 If `shuffle=true`, randomly permute the observations before splitting.
 A random number generator `rng` can be optionally passed as the first argument.