Document connection state transition timing

[Mirochill]

Summary

• clarify that received bytes are parsed by receive_data() while event-driven state changes are applied as events() is consumed
• document the remote-close case explicitly in both the generated API docs and the Connection method docstrings

Why

Issue #181 points out that it is easy to assume a remote close frame moves the connection into REMOTE_CLOSING during receive_data(), when the current implementation actually performs that transition while yielding the corresponding CloseConnection event from events().

Validation

• No local validation run, per this workspace's coordination-only policy.
• Relying on static review of the diff and the repository's remote CI.