Add warning about multiline centering the text on Android (facebook#735)

React Native has this issue where when using a `TextInput` with `multiline` set as `true`, the text is aligned to the top on iOS and to the center on Android by default, which causes confusion, evidenced by the amount of likes in [this comment](facebook/react-native#13897 (comment)). I believe the `textAlignVertical` default should be changed when `multiline` is `true`, but since I don't know if there are any reasons the issue I linked was closed, I suggest this is at least documented.

Author: filiptdz

docs/textinput.md

@@ -437,7 +437,7 @@ Limits the maximum number of characters that can be entered. Use this instead of
 
 ### `multiline`
 
-If `true`, the text input can be multiple lines. The default value is `false`.
+If `true`, the text input can be multiple lines. The default value is `false`. It is important to note that this aligns the text to the top on iOS, and centers it on Android. Use with `textAlignVertical` set to `top` for the same behavior in both platforms.
 
 | Type | Required |
 | ---- | -------- |