Clearer documentation wording & data model for usernames & handles

[matheus23]

Originally from comments from @hugomrdias:

we should describe better username, handle and domain in the context of this api

Where "this API" is:

• the _did.<handle> DNS records
• the GET /api/v0/account route and its returned Account record with the username field
• the PATCH /api/v0/account/username/:username route
• the PATCH /api/v0/account/handle/:handle route

some notes on username/handle :

• username and handle should be more explicit
  • username should just be username hugomrdias
  • handle should be hugomrdias.fission.codes
  • account info uses username as <username>.<domain>
  • patch username uses username as <username>
  • patch handle uses handle as just the domain