Update Light flash description

`light.turn_on` service description for the `flash` option gave the impression of a boolean value being required when in fact a string of either `short` or `long` was required.  Updated this to match the documentation found at https://www.home-assistant.io/integrations/light

`light.turn_off` also described the existence of a `flash` option when none exists.  I've removed this, which matches the aforementioned documentation too.

homeassistant/components/light/strings.json

@@ -308,7 +308,7 @@
         },
         "flash": {
           "name": "Flash",
-          "description": "If the light should flash."
+          "description": "Tell light to flash, can be either value short or long."
         },
         "effect": {
           "name": "Effect",
@@ -323,10 +323,6 @@
         "transition": {
           "name": "[%key:component::light::services::turn_on::fields::transition::name%]",
           "description": "[%key:component::light::services::turn_on::fields::transition::description%]"
-        },
-        "flash": {
-          "name": "[%key:component::light::services::turn_on::fields::flash::name%]",
-          "description": "[%key:component::light::services::turn_on::fields::flash::description%]"
         }
       }
     },