improve image and style doc
improve image and style doc

For me as a newbie, avatar and leftIcon was quite confusing.
These changes should make it clearer, that the props avatar and avatarStyle *are* in fact the IMage props source and style.
Also fixed some mixed-up object(style) and explained parallel use.
martinezguillaume/react-native-elements,kosiakMD/react-native-elements,kosiakMD/react-native-elements,react-native-community/React-Native-Elements