doc: cleanup and improve for recently moved pkgs (#354)

* chore(atomicx): review docs and add usage example

* chore(fsx): improve docs, return value, add examples

* fix(kvstore): correct typo and add example

* fix(multierror): add basic example

* doc: revamp ooapi documentation

internal/multierror/example_test.go

@@ -0,0 +1,16 @@
+package multierror_test
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/ooni/probe-cli/v3/internal/multierror"
+)
+
+func ExampleUnion() {
+	root := errors.New("some error")
+	me := multierror.New(root)
+	check := errors.Is(me, root)
+	fmt.Printf("%+v\n", check)
+	// Output: true
+}