Fix UGate OpenQASM 3 phase note

[TSS99]

Summary

Correct the UGate docstring note that compares Qiskit's matrix convention with the current OpenQASM 3 U definition.

Fixes #15830.

Problem

The existing note says Qiskit's displayed UGate matrix is the same as OpenQASM 3. The current OpenQASM 3 specification defines U with an additional global phase relative to Qiskit's matrix.

Root cause

The docstring appears to describe an older OpenQASM 3 convention. The live OpenQASM 3 specification now defines U(theta, phi, lambda) as Qiskit's matrix multiplied by exp(i * theta / 2).

Fix

Update the note to state the current OpenQASM 3 global-phase relationship, while preserving the existing note about earlier OpenQASM definitions.

Tests

• .venv/bin/python -m black --check qiskit/circuit/library/standard_gates/u.py
• .venv/bin/python -m ruff check qiskit/circuit/library/standard_gates/u.py
• Verified numerically that the current OpenQASM 3 formula equals exp(i * theta / 2) times Qiskit's UGate matrix for sampled parameters.

Notes / limitations

This is a documentation-only correction; no runtime behavior changes.

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description: OpenAI Codex (GPT-5)
• I used the following tool to generate or modify code: OpenAI Codex (GPT-5)

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