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/client.go

@@ -3,11 +3,16 @@ package ooapi
 // Client is a client for speaking with the OONI API. Make sure you
 // fill in the mandatory fields.
 type Client struct {
-	BaseURL      string       // optional
-	GobCodec     GobCodec     // optional
-	HTTPClient   HTTPClient   // optional
-	JSONCodec    JSONCodec    // optional
-	KVStore      KVStore      // mandatory
-	RequestMaker RequestMaker // optional
-	UserAgent    string       // optional
+	// KVStore is the MANDATORY key-value store. You can use
+	// the kvstore.Memory{} struct for an in-memory store.
+	KVStore KVStore
+
+	// The following fields are optional. When they are empty
+	// we will fallback to sensible defaults.
+	BaseURL      string
+	GobCodec     GobCodec
+	HTTPClient   HTTPClient
+	JSONCodec    JSONCodec
+	RequestMaker RequestMaker
+	UserAgent    string
 }