gh-132819: Clarify socket.connect() signal interruption behavior

[Taeknology]

Clarify how socket.connect() behaves when a connection attempt is
interrupted by a signal. The documentation now separates signal-handler
exceptions, blocking and timeout modes, and non-blocking behavior.

Add Linux-specific tests covering retry after a non-raising signal handler and
propagation when the signal handler raises an exception.

Closes #132819.

Tests

• ./python -m test test_socket
• make -C Doc html SPHINXERRORHANDLING=-W

• Issue: Can the explanation of socket.connect() connection failures be rephrased? I don't think anybody understands what's written. #132819

[read-the-docs-community]

Documentation build overview

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± library/socket.html