Move the default docstring construction to occur on-access through a descriptor.

Verified with [tprof](https://github.com/adamchainz/tprof) and this script that generates 10k dataclasses:

```py
from dataclasses import dataclass

for i in range(10_000):
    @DataClass
    class Example:
        field1: int
        field2: str
        field3: float
```

**Before:**

```
$ tprof -t dataclasses._process_class example.py
🎯 tprof results:
 function                     calls total  mean ± σ       min … max
 dataclasses._process_class() 10000    5s 485μs ± 120μs 458μs … 6ms
 ```

After:

```
$ PYTHONPATH=Lib/ uvx tprof -t dataclasses._process_class example.py
🎯 tprof results:
 function                     calls total  mean ± σ       min … max
 dataclasses._process_class() 10000    3s 275μs ± 131μs 245μs … 6ms
```

The mean time spent in `_process_class()` has dropped from 485μs to 275μs, a ~42% time saving (admittedly skewed due to the small size of the dataclass).