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