docs: show unaligned amplitudes in spin alignment page (#321)

* docs: add warning about missing λ=0

docs/usage/helicity/spin-alignment.ipynb

@@ -89,6 +89,7 @@
     "from IPython.display import Math\n",
     "\n",
     "import ampform\n",
+    "from ampform.helicity import HelicityModel\n",
     "from ampform.io import aslatex, improve_latex_rendering\n",
     "\n",
     "LOGGER = logging.getLogger()\n",
@@ -166,6 +167,41 @@
     "non_aligned_model.intensity"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "The symbols for the amplitudes are defined through {attr}`.HelicityModel.amplitudes`:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "jupyter": {
+     "source_hidden": true
+    },
+    "tags": [
+     "full-width",
+     "hide-input"
+    ]
+   },
+   "outputs": [],
+   "source": [
+    "def render_amplitudes(model: HelicityModel) -> Math:\n",
+    "    selected_amplitudes = {\n",
+    "        symbol: expr\n",
+    "        for i, (symbol, expr) in enumerate(model.amplitudes.items())\n",
+    "        if i % 5 == 0\n",
+    "    }\n",
+    "    src = aslatex(selected_amplitudes)\n",
+    "    src = src.replace(R\"\\end{array}\", R\"& \\dots \\\\ \\end{array}\")\n",
+    "    return Math(src)\n",
+    "\n",
+    "\n",
+    "render_amplitudes(non_aligned_model)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -179,7 +215,11 @@
    "source": [
     "One way of aligning the spins of each sub-system, is Dalitz-Plot Decomposition (DPD) {cite}`mikhasenkoDalitzplotDecompositionThreebody2020`. DPD **can only be used for three-body decays**, but results in a quite condense amplitude model expression.\n",
     "\n",
-    "We can select DPD alignment as follows:"
+    "We can select DPD alignment as follows:\n",
+    "\n",
+    ":::{warning}\n",
+    "The {class}`.DalitzPlotDecomposition` is not yet fully functional for reactions with a polarized initial or final state. In this example, the sums inside the incoherent sum should also include $\\lambda_0=0$.\n",
+    ":::"
    ]
   },
   {
@@ -260,6 +300,34 @@
     "Math(src)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Note that the amplitudes are the same as those in the non-aligned model:\n",
+    "\n",
+    ":::{warning}\n",
+    "This behavior is a bug that will be fixed through [ComPWA/ampform#318](https://github.com/ComPWA/ampform/issues/318).\n",
+    ":::"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "jupyter": {
+     "source_hidden": true
+    },
+    "tags": [
+     "full-width",
+     "hide-input"
+    ]
+   },
+   "outputs": [],
+   "source": [
+    "render_amplitudes(dpd_model)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {