Mention that `multiprocessing.connection.Connection.send` (and friends) can block

[pohlt]

The documentation for multiprocessing.connection.Connection.send doesn't mention that the call might block even for rather small messages.

The same is true for

• multiprocessing.connection.Connection.send_bytes and
• multiprocessing.SimpleQueue.put.

Judging by StackOverflow I'm not the only one who got bitten.