Add specs for API methods to retrieve the users nicknames of their own accounts

Dapps may wish retrieve a user's nicknames for their own accounts, so that they can represent those accounts in the same way that the wallet does

Start by reviewing the repository's existing API method specifications and how related account methods are documented. Define the methods needed to retrieve users' nicknames for their own accounts, then confirm the specifications let dapps represent those accounts as the wallet does.

Written by the indexing model from the issue text.