bug: async cached functions use ainvalidate_cache() not cache_clear()

[27Bslash6]

Description

The API for clearing cache on async decorated functions is inconsistent with sync functions.

Evidence

# Sync — cache_clear() works
@cache(backend=None, ttl=300)
def sync_fn(x): return x * 2
sync_fn.cache_clear()  # ✓

# Async — cache_clear() raises TypeError, must use ainvalidate_cache()
@cache(backend=None, ttl=300)
async def async_fn(x): return x * 2
async_fn.cache_clear()  # TypeError
await async_fn.ainvalidate_cache()  # ✓ (must be awaited)

Impact

• Inconsistent API surface between sync and async decorators
• Users expect cache_clear() to work on all decorated functions (it's stdlib convention from lru_cache)
• Not documented anywhere

Suggested Fix

Either:

1. Make cache_clear() work synchronously on async functions too (preferred — matches lru_cache convention)
2. Or document the ainvalidate_cache() requirement prominently in API reference and getting-started

Found by: tests/competitive/test_head_to_head.py::TestAsyncSupport