Update the docstring for the choosetests function

Author: DilumAluthge

test/choosetests.jl

@@ -33,15 +33,15 @@ const TESTNAMES = [
 
 """
 
-`tests, net_on, exit_on_error, seed = choosetests(choices)` selects a set of tests to be
+`(; tests, net_on, exit_on_error, seed) = choosetests(choices)` selects a set of tests to be
 run. `choices` should be a vector of test names; if empty or set to
 `["all"]`, all tests are selected.
 
 This function also supports "test collections": specifically, "linalg"
  refers to collections of tests in the correspondingly-named
 directories.
 
-Upon return:
+The function returns a named tuple with the following elements:
   - `tests` is a vector of fully-expanded test names,
   - `net_on` is true if networking is available (required for some tests),
   - `exit_on_error` is true if an error in one test should cancel