Replace variadic argument to all AddDocuments and UpdateDocuments methods

[alallema]

Description
Currently, we use variable parameters for all methods related to AddDocuments and UpdateDocuments to simplify the use of these methods. But this is not a good practice and it would be better to declare with normal parameters even if the user has to send nil for some of the unused parameters.

Basic example
Such as the method AddDocuments:

	AddDocuments(documentsPtr interface{}, primaryKey ...string) (resp *TaskInfo, err error)

Should become:

	AddDocuments(documentsPtr interface{}, primaryKey string) (resp *TaskInfo, err error)

And used as:

  documents := []map[string]interface{}{
    {
      "id":     287947,
      "title":  "Shazam ⚡️",
      "genres": "comedy",
    },
  }
  client.Index("movies").AddDocuments(documents, nil)

Other

• Adapt the code of the methods
• Adapt the tests
• Update the code sample to the new usage