Updating note on U-gate deprecation Qiskit#1330 (Qiskit#1333)

* Updating note on U-gate deprecation Qiskit#1330

* Typo update

* Adding in formatting and code example

* Trying different html formatting

* fix alert info box

* split code

* Revert "split code"

This reverts commit d044496.

* improve format

* use bullet points

Co-authored-by: Junye Huang <h.jun.ye@gmail.com>

tutorials/circuits/3_summary_of_quantum_operations.ipynb

@@ -228,6 +228,34 @@
     "job.result().get_unitary(qc, decimals=3)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "<div class=\"alert alert-info\">\n",
+    "Note on U gate deprecation\n",
+    "\n",
+    "The QuantumCircuit methods $u1$, $u2$ and $u3$ are now deprecated. Instead, the following replacements should be used.\n",
+    "\n",
+    "- $u1(\\lambda) = p(\\lambda) = u(0, 0, \\lambda)$\n",
+    "\n",
+    "- $u2(\\phi, \\lambda) = u(\\frac{\\pi}{2}, \\phi, \\lambda) = p(\\frac{\\pi}{2} + \\phi) \\cdot sx \\cdot p(\\frac{\\pi}{2} - \\lambda)$\n",
+    "\n",
+    "- $u3(\\theta, \\phi, \\lambda) = u(\\theta, \\phi, \\lambda) = p(\\phi + \\pi) \\cdot sx \\cdot p(\\theta + \\pi) \\cdot sx \\cdot p(\\lambda)$\n",
+    "\n",
+    "```python\n",
+    "# qc.u1(lambda) is now:\n",
+    "qc.p(lambda)\n",
+    "\n",
+    "# qc.u2(phi, lambda) is now:\n",
+    "qc.u(pi/2, phi, lambda)\n",
+    "\n",
+    "# qc.u3(theta, phi, lambda) is now:\n",
+    "qc.u(theta, phi, lambda)\n",
+    "```\n",
+    "</div>"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},