docs: clarify maintainer name format

    where

    - `handle` is the handle you are going to use in nixpkgs expressions,

    - `name` is your, preferably real, name,

    - `name` is a name that people would know and recognize you by,

    - `email` is your maintainer email address,

    - `matrix` is your Matrix user ID,

    - `github` is your GitHub handle (as it appears in the URL of your profile page, `https://github.com/<userhandle>`),