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/ooapi/example_test.go

@@ -0,0 +1,36 @@
+package ooapi_test
+
+import (
+	"context"
+	"fmt"
+	"log"
+
+	"github.com/ooni/probe-cli/v3/internal/kvstore"
+	"github.com/ooni/probe-cli/v3/internal/ooapi"
+	"github.com/ooni/probe-cli/v3/internal/ooapi/apimodel"
+)
+
+func ExampleClient() {
+	clnt := &ooapi.Client{
+		KVStore: &kvstore.Memory{},
+	}
+	ctx := context.Background()
+	resp, err := clnt.CheckIn(ctx, &apimodel.CheckInRequest{
+		Charging:        false,
+		OnWiFi:          false,
+		Platform:        "linux",
+		ProbeASN:        "AS30722",
+		ProbeCC:         "IT",
+		RunType:         "timed",
+		SoftwareName:    "miniooni",
+		SoftwareVersion: "0.1.0-dev",
+		WebConnectivity: apimodel.CheckInRequestWebConnectivity{
+			CategoryCodes: []string{"NEWS"},
+		},
+	})
+	fmt.Printf("%+v\n", err)
+	// Output: <nil>
+	if resp == nil {
+		log.Fatal("expected non-nil response")
+	}
+}