add explanation why they don't need :A in namelist exercise

Author: cecilehannay

notebooks/namelist/exercises/exercise_cam_output.ipynb

@@ -138,6 +138,8 @@
     " fincl2 = 'T:I','Q:I','U:I','V:I'\n",
     " fincl3 = 'T','Q','U','V'\n",
     "```\n",
+    "\n",
+    "💡 Note: For `fincl3`, you could write `fincl3 = 'T:A','Q:A','U:A','V:A'`.  But since averaging (:A) is the default for these variables, it's not necessary to specify it explicitly.\n",
     " \n",
     "**# Set run length**\n",
     "    \n",