Clarify in documentation the saturating min requires specifying cap

docs/_docs/saturating_forecasts.md

@@ -64,6 +64,8 @@ m.plot(fcst);
 ![png](/prophet/static/saturating_forecasts_files/saturating_forecasts_13_0.png) 
 
 
+The logistic function has an implicit minimum of 0, and will saturate at 0 the same way that it saturates at the capacity. It is possible to also specify a different saturating minimum.
+
 ### Saturating Minimum
 
 The logistic growth model can also handle a saturating minimum, which is specified with a column `floor` in the same way as the `cap` column specifies the maximum:
@@ -94,3 +96,5 @@ m.plot(fcst);
  
 ![png](/prophet/static/saturating_forecasts_files/saturating_forecasts_16_0.png) 
 
+
+To use a logistic growth trend with a saturating minimum, a maximum capacity must also be specified.

notebooks/saturating_forecasts.ipynb

@@ -222,6 +222,8 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
+    "The logistic function has an implicit minimum of 0, and will saturate at 0 the same way that it saturates at the capacity. It is possible to also specify a different saturating minimum.\n",
+    "\n",
     "### Saturating Minimum\n",
     "\n",
     "The logistic growth model can also handle a saturating minimum, which is specified with a column `floor` in the same way as the `cap` column specifies the maximum:"
@@ -294,6 +296,13 @@
     "fcst = m.predict(future)\n",
     "m.plot(fcst);"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "To use a logistic growth trend with a saturating minimum, a maximum capacity must also be specified."
+   ]
   }
  ],
  "metadata": {