Doc: document --color-separator

Author: CarterLi

src/data/help.json

@@ -457,6 +457,14 @@
                 "type": "color"
             }
         },
+        {
+            "long": "color-separator",
+            "desc": "Set the color of the key-value separator",
+            "remark": "See `-h color` for the list of available colors",
+            "arg": {
+                "type": "color"
+            }
+        },
         {
             "long": "color",
             "desc": "Set the color of both the keys and title",