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/fsx/example_test.go

@@ -0,0 +1,34 @@
+package fsx_test
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"path/filepath"
+	"syscall"
+
+	"github.com/ooni/probe-cli/v3/internal/fsx"
+)
+
+func ExampleOpenFile_openingDir() {
+	filep, err := fsx.OpenFile("testdata")
+	if !errors.Is(err, syscall.ENOENT) {
+		log.Fatal("unexpected error", err)
+	}
+	if filep != nil {
+		log.Fatal("expected nil fp")
+	}
+}
+
+func ExampleOpenFile_openingFile() {
+	filep, err := fsx.OpenFile(filepath.Join("testdata", "testfile.txt"))
+	if err != nil {
+		log.Fatal("unexpected error", err)
+	}
+	data, err := io.ReadAll(filep)
+	if err != nil {
+		log.Fatal("unexpected error", err)
+	}
+	fmt.Printf("%d\n", len(data))
+}