[ReactNative] Text docs

Summary:
In preparation for open sourcing React Native for Android, document which Text props are platform-specific.

Libraries/Text/TextStylePropTypes.js

@@ -16,30 +16,52 @@ var ViewStylePropTypes = require('ViewStylePropTypes');
 
 // TODO: use spread instead of Object.assign/create after #6560135 is fixed
 var TextStylePropTypes = Object.assign(Object.create(ViewStylePropTypes), {
+  color: ReactPropTypes.string,
   fontFamily: ReactPropTypes.string,
   fontSize: ReactPropTypes.number,
+  fontStyle: ReactPropTypes.oneOf(['normal', 'italic']),
+  /**
+   * Specifies font weight. The values 'normal' and 'bold' are supported for
+   * most fonts. Not all fonts have a variant for each of the numeric values,
+   * in that case the closest one is chosen.
+   */
   fontWeight: ReactPropTypes.oneOf(
     ['normal' /*default*/, 'bold',
      '100', '200', '300', '400', '500', '600', '700', '800', '900']
   ),
-  fontStyle: ReactPropTypes.oneOf(['normal', 'italic']),
+  /**
+   * @platform ios
+   */
+  letterSpacing: ReactPropTypes.number,
   lineHeight: ReactPropTypes.number,
-  color: ReactPropTypes.string,
-  // NOTE: 'justify is supported only on iOS
+  /**
+   * Specifies text alignment. The value 'justify' is only supported on iOS.
+   */
   textAlign: ReactPropTypes.oneOf(
     ['auto' /*default*/, 'left', 'right', 'center', 'justify']
   ),
+  /**
+   * @platform ios
+   */
+  textDecorationLine: ReactPropTypes.oneOf(
+    ['none' /*default*/, 'underline', 'line-through', 'underline line-through']
+  ),
+  /**
+   * @platform ios
+   */
+  textDecorationStyle: ReactPropTypes.oneOf(
+    ['solid' /*default*/, 'double', 'dotted','dashed']
+  ),
+  /**
+   * @platform ios
+   */
+  textDecorationColor: ReactPropTypes.string,
+  /**
+   * @platform ios
+   */
   writingDirection: ReactPropTypes.oneOf(
     ['auto' /*default*/, 'ltr', 'rtl']
   ),
-  letterSpacing: ReactPropTypes.number,
-  textDecorationLine:ReactPropTypes.oneOf(
-    ['none' /*default*/, 'underline', 'line-through', 'underline line-through']
-  ),
-  textDecorationStyle:ReactPropTypes.oneOf(
-    ['solid' /*default*/, 'double', 'dotted','dashed']
-  ),
-  textDecorationColor: ReactPropTypes.string,
 });
 
 // Text doesn't support padding correctly (#4841912)