doc(readme): StatusBar.backgroundColorByHexString only supports #RRGGBB on iOS

[GitToTheHub]

Platforms affected

• iOS

Motivation and Context

The documentation for iOS about StatusBar.backgroundColorByHexString assumed, that an alpha value could be possible, when StatusBar.overlaysWebView is true. Or the documentation wanted to assume, that the background color can only be set if StatusBar.overlaysWebView is true. To avoid confusion, the documentation just states, that iOS supports only the format #RRGGBB.

On the Android part the WP7&8 part was removed since Window Phone is not supported by this plugin anymore.

Description

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[GitToTheHub]

Superseded by #293