[docs] add a note that registerModule can receive an array of string

[ktsn]

Fix #633

[posva]

I think it'll be better to directly provide an example (in addition to the existing one). Something like:

  // register a nested module nested/module
  store.registerModule(['nested', 'module'], {
    // ...
  })

[ktsn]

@posva Thanks! I've amended it.