Improve pool documentation examples (#491)

I think pool documentation should recommend the safest approaches first (i.e. with the fewest possible mistakes), and discourage lower-level approach.

Author: nyurik

asyncpg/pool.py

@@ -798,14 +798,27 @@ def create_pool(dsn=None, *,
 
  Can be used either with an ``async with`` block:
 
+ .. code-block:: python
+
+ async with asyncpg.create_pool(user='postgres',
+ command_timeout=60) as pool:
+ await pool.fetch('SELECT 1')
+
+ Or to perform multiple operations on a single connection:
+
  .. code-block:: python
 
  async with asyncpg.create_pool(user='postgres',
  command_timeout=60) as pool:
  async with pool.acquire() as con:
+ await con.execute('''
+ CREATE TABLE names (
+ id serial PRIMARY KEY,
+ name VARCHAR (255) NOT NULL)
+ ''')
  await con.fetch('SELECT 1')
 
- Or directly with ``await``:
+ Or directly with ``await`` (not recommended):
 
  .. code-block:: python