Document exceptions

Please add the asyncpg exceptions to the documentation. I have no trouble reading source code, so was pleasantly surprised to find how rich the included exception classes are.

```python
>>> exceptions = sorted((o for o in vars(asyncpg).values() if isinstance(o, type) and issubclass(o, Exception)), key=lambda o: o.__name__)
>>> for exc in exceptions:
... if getattr(exc, 'sqlstate', None): continue # looong list otherwise
... print(exc.__name__)
... if exc.__doc__: print(exc.__doc__)
... print()
...
FatalPostgresError
A fatal error that should result in server disconnection.

InterfaceError
An error caused by improper use of asyncpg API.

InterfaceWarning
A warning caused by an improper use of asyncpg API.

InternalClientError
All unexpected errors not classified otherwise.

OutdatedSchemaCacheError
A value decoding error caused by a schema change before row fetching.

PostgresError
Base class for all Postgres errors.

ProtocolError
Unexpected condition in the handling of PostgreSQL protocol input.

UnknownPostgresError
An error with an unknown SQLSTATE code.

>>> # generated exceptions for all possible sqlstate values
...
>>> sum(1 for exc in exceptions if getattr(exc, 'sqlstate', None))
240
```

Start from the asyncpg exception classes and the exception list shown in the issue, separating the base and API exceptions from the SQLSTATE-generated exceptions. Document the available classes and their descriptions, including the SQLSTATE coverage; done means the exception reference is covered in the project documentation.

Written by the indexing model from the issue text.