doc: i2p: improve `-i2pacceptincoming` mention #29813

In i2p documentation, it says that "the first time Bitcoin Core connects to the I2P router, it automatically generates a persistent I2P address and its corresponding private key by default or if -i2pacceptincoming=1 is set". This is weird, because -i2pacceptincoming=1 by itself does not have any effect. Moreover, -i2pacceptincoming is 1 by default anyway.

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cc: @vasild

This documentation change is correct and makes the documentation slightly shorter, thus easier to read. ACK 2179e2c3209a41c1419f1f5ed6270a0dad68b50d

ACK 2179e2c3209a41c1419f1f5ed6270a0dad68b50d

ACK https://github.com/bitcoin/bitcoin/pull/29813/commits/2179e2c3209a41c1419f1f5ed6270a0dad68b50d

Checked (without testing) that behavior is as described. In CConman::ConnectNode, unless i2pacceptincoming == 0, a persistent I2P session (m_transient == false) with a private key stored on disk is used. Otherwise, we create a transient I2P session (m_transient == true).

Relevant section of i2p::sam::Session::CreateIfNotCreatedAlready() (called by i2p::sam::Session::Connect):

if (m_transient) {
	// The destination (private key) is generated upon session creation and returned
	// in the reply in DESTINATION=.

	// [...]
} else {
	// Read our persistent destination (private key) from disk or generate
	// one and save it to disk. Then use it when creating the session.
	const auto& [read_ok, data] = ReadBinaryFile(m_private_key_file);
	if (read_ok) {
		m_private_key.assign(data.begin(), data.end());
	} else {
		GenerateAndSavePrivateKey(*sock);
	}
	// [...]
}

ACK 2179e2c3209a41c1419f1f5ed6270a0dad68b50d

ACK 2179e2c3209a41c1419f1f5ed6270a0dad68b50d