Cannot (should not?) set isolation level on connection?

I find in the documentation here https://aiopg.readthedocs.io/en/stable/core.html?highlight=isolation_level#aiopg.Connection.isolation_level

`The only value allowed in asynchronous mode value is psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED (READ COMMITTED).`

However, I am not able to find in the psycopg2 documentation where this is stated. http://initd.org/psycopg/docs/advanced.html#async-support

`With asynchronous connections it is also not possible to use set_client_encoding(), executemany(), large objects, named cursors.`

Additionally the docs state `Similarly set_session() can’t be used but it is still possible to invoke the SET command with the proper default_transaction_... parameter.`

`aiopg` allows `set_isolation_level` to be called on the connection object with any value and does not check for 'READ_COMMITTED' explicitly, but this "warning" still exists in the source and documentation.

Why is this? Can the documentation link to the relevant section of the psycopg2 docs?