Update overflow docstring to add note on Android

Summary:
The overflow: visible option does not work on Android. This just add a little note to the docs of that property to make that clear.

See #3198  and https://react-native.canny.io/feature-requests/p/add-overflow-support-to-android .

Make it clear that this property is unavailable on Android. We had to figure this out by trial and error when porting an existing codebase, a small note in the docs would be helpful!

I could not find how to build the website - I tried `cd website && npm install && node ./server/generate.js` but I'm getting an error (*JSDOC_ERROR: There are no input files to process.*). But it is such a small change I can't see how it would break the docs build!
Closes https://github.com/facebook/react-native/pull/14103

Differential Revision: D5225814

Pulled By: shergin

fbshipit-source-id: a3136e791178d2a7090f324041be23628130199e

Libraries/StyleSheet/LayoutPropTypes.js

@@ -429,6 +429,8 @@ var LayoutPropTypes = {
    *  It works like `overflow` in CSS (default: visible).
    *  See https://developer.mozilla.org/en/docs/Web/CSS/overflow
    *  for more details.
+   *  `overflow: visible` only works on iOS. On Android, all views will clip
+   *  their children.
    */
   overflow: ReactPropTypes.oneOf([
     'visible',