Update mixin docs

Author: bytesnz

docs/mixins/circle.md

@@ -1,6 +1,11 @@
 ### Props
 ```js
 {
+  fill: {
+    type: Boolean,
+    custom: true,
+    default: true
+  },
   radius: {
     type: Number,
     default: null

docs/mixins/path.md

@@ -49,7 +49,7 @@
   fill: {
     type: Boolean,
     custom: true,
-    default: true
+    default: false
   },
   fillColor: {
     type: String,

docs/mixins/polygon.md

@@ -1,6 +1,13 @@
 ### Props
-
-`Polygon` does not have any props
+```js
+{
+  fill: {
+    type: Boolean,
+    custom: true,
+    default: true
+  }
+}
+```
 
 ### Methods