internal/tdtest: document how the name field is used

This detail wasn't clear from the doc comments.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I92f70df872244e6379692ddb756dbde1bd26ecd9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1170245
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

internal/tdtest/tdtest.go

@@ -62,6 +62,8 @@ type set[TC any] struct {
 }
 
 // Run runs the given function for each (selected) element in the table.
+// TC must be a struct type. If that has a string field named "name",
+// that value will be used to name the associated subtest.
 func Run[TC any](t *testing.T, table []TC, fn func(t *T, tc *TC)) {
 	s := &set[TC]{
 		t:             t,