Documentation references non-existent `AbstractAsyncFeatureCache` class

## Description

The Python SDK documentation at https://docs.growthbook.io/lib/python shows examples using `AbstractAsyncFeatureCache` for custom async caching, but this class doesn't exist in the latest released version (1.4.7).

## Documentation Example

The docs show this example for custom async caching:

```python
from redis.asyncio import Redis
from growthbook import GrowthBookClient, Options, AbstractAsyncFeatureCache

class AsyncRedisFeatureCache(AbstractAsyncFeatureCache):
def __init__(self):
self.redis = Redis(host='localhost', port=6379, decode_responses=True)
self.prefix = "gb:async:"

async def get(self, key: str):
data = await self.redis.get(self.prefix + key)
return None if data is None else json.loads(data)

async def set(self, key: str, value: dict, ttl: int) -> None:
await self.redis.set(
self.prefix + key,
json.dumps(value),
ex=ttl
)

async def close(self):
await self.redis.close()

# Create client with custom cache
client = GrowthBookClient(
Options(
api_host="https://cdn.growthbook.io",
client_key="sdk-abc123",
cache=cache # <-- Options doesn't support 'cache' parameter
)
)
```

## Actual Behavior

When attempting to use this code with growthbook==1.4.7:

```python
from growthbook import AbstractAsyncFeatureCache
# ImportError: cannot import name 'AbstractAsyncFeatureCache' from 'growthbook'
```

Additionally, the `Options` class doesn't have a `cache` parameter.

## Investigation

Checking the installed package:

```bash
$ pip show growthbook
Name: growthbook
Version: 1.4.7

$ python -c "import growthbook; print([x for x in dir(growthbook) if 'Async' in x or 'Cache' in x])"
['AbstractFeatureCache', 'CacheEntry', 'FeatureCache', 'InMemoryFeatureCache']
# No AbstractAsyncFeatureCache
```

The package only contains:
- `AbstractFeatureCache` (sync, for legacy `GrowthBook` class)
- `FeatureCache` (internal class used by `GrowthBookClient`)
- `InMemoryFeatureCache` (sync implementation)

## Expected Behavior

Either:
1. Documentation should be updated to remove references to `AbstractAsyncFeatureCache` and custom async caching for version 1.4.7
2. Or the documentation should indicate this is a feature coming in a future version (e.g., "Available in v2.0+")
3. Or `AbstractAsyncFeatureCache` should be added to the library to match the documentation

## Workaround

For now, custom caching is only supported for the synchronous client:

```python
from growthbook import AbstractFeatureCache, feature_repo

class RedisFeatureCache(AbstractFeatureCache):
# Sync implementation...
pass

# Configure globally for sync client
feature_repo.set_cache(RedisFeatureCache())
```

## Environment

- Python: 3.12
- growthbook: 1.4.7
- OS: macOS/Linux

## Additional Context

This issue was discovered while implementing Redis caching for a Django application following the official documentation. The documentation appears to be written for a future or unreleased version of the SDK.

Would appreciate clarification on:
1. When will `AbstractAsyncFeatureCache` be available?
2. Should we expect this in a minor or major version update?
3. Can the documentation be tagged with version availability?

Thank you!