way to document TypeVars


When I have a class like this:

```python3
class MyGeneric[T]:
    ...
```

or the legacy-syntax equivalent:

```python3
from typing import TypeVar, Generic
T = TypeVar("T")
class MyGeneric(Generic[T]):
    ...
```

"`T`" is a variable bound in the scope of `MyGeneric`, which needs documenting as much as any `@param` or `@ivar` does. I would suggest `@tvar` to describe a type variable?