docs(multichannel): clarify channel selection is by name (int only for integer-named axes)

Author: timtreis

tutorials/multichannel_images.ipynb

@@ -148,7 +148,9 @@
    "source": [
     "## 2. A single channel — `channel=`\n",
     "\n",
-    "Pass a channel **name** (or index) to render just that marker. `DAPI` stains nuclei."
+    "Pass a channel **name** to render just that marker — channels are selected by name. (An integer\n",
+    "works only when the channel axis is itself integer-named, as in the synthetic `blobs` image; the\n",
+    "named markers here must be selected by name.) `DAPI` stains nuclei."
    ]
   },
   {