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/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()