improve bevy_ui `Direction` docs

[adsick]

not a big deal, but the doc comments are talking about text direction, while Style.direction is about content direction.

[nicoburns]

I don't think bevy actually implements the content direction part of the direction property.

In my CSS Grid PR I have updated the descriptions all the style props and have written the following about direction:

/// Defines the text direction. For example English is written LTR (left-to-right) while Arabic is written RTL (right-to-left).
///
/// Note: the corresponding CSS property also affects box layout order, but this isn't yet implemented in bevy.
/// <https://developer.mozilla.org/en-US/docs/Web/CSS/direction>

[ickshonpe]

I made an example for direction ages ago, I'll make a PR it for after the game jam is over.

[doup]

I think this one can be closed since direction was removed.