Add descriptions to models, parameters, fields

[ehmicky]

Adding descriptions to the following would be very useful for developers:

• endpoints (operations):
  • summary (short description)
  • description (long description)
  • externalDocs (link to online documentation)
• request parameters
• request payloads
• models / schemas
• security schemes
• tags (description and externalDocs)

Having only the attribute names is useful but does not provide enough context.