Note which kwargs are safe to pass to Pool.set_connect_args

The documentation for asyncpg.pool.Pool.set_connect_args makes no mention that there are 2 particular kwargs that are already being passed to connect by the pool when it creates new connections: https://github.com/MagicStack/asyncpg/blob/92c2d81256a1efd8cab12c0118d74ccd1c18131b/asyncpg/pool.py#L459-L463

Before reading the source, I incorrectly assumed that any of connect's kwargs are safe to pass to Pool.set_connect_args, but if you pass either loop or connection_class you'll end up with a traceback like this:

Traceback (most recent call last):
... omitted for brevity ...
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 762, in __aenter__
self.connection = await self.pool._acquire(self.timeout)
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 604, in _acquire
return await _acquire_impl()
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 589, in _acquire_impl
proxy = await ch.acquire() # type: PoolConnectionProxy
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 133, in acquire
await self.connect()
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 125, in connect
self._con = await self._pool._get_new_connection()
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 463, in _get_new_connection
**self._connect_kwargs)
TypeError: connect() got multiple values for keyword argument 'loop'

Not hard to fix in my app, but it'd be great to update the docs to maybe save future users some time.

从链接的 asyncpg.pool.Pool.set_connect_args API 文档以及 asyncpg/pool.py 中引用的第 459-463 行开始。更新文档，将 loop 和 connection_class 标识为已由 pool 提供且无法安全传入的 kwargs。验证生成的 API 文档，并确保警告与观察到的 TypeError 一致。