refactor: improve the accuracy of docs/improve docs

[NotSugden]

Please describe the changes this PR makes and why it should be merged:
This PR changes some code to

• match the documentation, some properties are typed as ?Type which indicates nullable but were being set to undefined, they are now set to null
  • this also just changes some code in general to prefer null over undefined
• correctly types some properties as nullable (such as Guild#voice)
• removes unnessercary @name tags from certian declarations, they are only needed if the typedef is not directly above the actual declaration
• changes a couple of @prop tags to @property for consistency
• changes some classes to have an @abstract tag on them, as they are abstract classes

this PR is a breaking change

Status

• Code changes have been tested against the Discord API, or there are no code changes
  • not fully tested if null causes any internal problems with the library (eg `typeof x === 'undefined'), will get round to it
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.