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/kvstore/example_test.go

@@ -0,0 +1,27 @@
+package kvstore_test
+
+import (
+	"errors"
+	"fmt"
+	"log"
+	"reflect"
+
+	"github.com/ooni/probe-cli/v3/internal/kvstore"
+)
+
+func ExampleMemory() {
+	kvs := &kvstore.Memory{}
+	if _, err := kvs.Get("akey"); !errors.Is(err, kvstore.ErrNoSuchKey) {
+		log.Fatal("unexpected error", err)
+	}
+	val := []byte("value")
+	if err := kvs.Set("akey", val); err != nil {
+		log.Fatal("unexpected error", err)
+	}
+	out, err := kvs.Get("akey")
+	if err != nil {
+		log.Fatal("unexpected error", err)
+	}
+	fmt.Printf("%+v\n", reflect.DeepEqual(val, out))
+	// Output: true
+}

internal/kvstore/memory.go

@@ -30,7 +30,7 @@ func (kvs *Memory) Get(key string) ([]byte, error) {
 	return value, nil
 }
 
-// Set sets a key into the key-value store
+// Set sets a key into the key-value store.
 func (kvs *Memory) Set(key string, value []byte) error {
 	kvs.mu.Lock()
 	defer kvs.mu.Unlock()