Update rainbow bbcode options - add speed

[havi05]

godotengine/godot#100494 added a new option to the bbcode effect rainbow called speed to fix the animation direction. Updated the documentation to include this.

[skyace65]

Thanks a ton! We really appreciate it when engine contributors document their PRs

[havi05]

Since the mentioned pr is considered for cherry-picking, do I also have to create pr's for these branches (4.3, 4.2) or will this be done by the maintainers if needed?

Thanks for the kind words!

Edit: Found the answer in the docs.

[tetrapod00]

@havi05 Yeah, you don't need to take any more actions.

To clarify a little more, while the engine PR is marked for cherrypicking, so far it has not been cherrypicked, and therefore the documentation PR isn't marked for cherrypicking. We wouldn't want to document features that don't exist yet. Right now we usually only cherrypick back a single minor version, so there is a chance that the documentation for this feature will only ever exist in the 4.4 and forward version of the documentation, depending on the timing of everything.