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/atomicx/example_test.go

@@ -0,0 +1,14 @@
+package atomicx_test
+
+import (
+	"fmt"
+
+	"github.com/ooni/probe-cli/v3/internal/atomicx"
+)
+
+func Example_typicalUsage() {
+	v := &atomicx.Int64{}
+	v.Add(1)
+	fmt.Printf("%d\n", v.Load())
+	// Output: 1
+}