`mailchimp` improvements

In Summary

Most of the function available in mailchimp adaptor does not have a documentation that correspond to a Mailchimp API endpoints. which make it hard to understand which adaptor function to use.

Request in simple words as possible

• Add description in each mailchimp function, that correspond with Mailchimp API endpoint that is used behind that function
• Document the supported options for each function
• Add examples of how to use the function if possible
• Update function names so that they clearly describe the operation (i.e., we know the existing function updateMember() needs to be changed to -> upsertMember() because it both creates and updates members)
• update upsertMembers to use members as option not users