[swift] Rename compose method and concept in docs to render

[zach-klippenstein]

The compose function was originally called render in the Swift implementation. After much discussion, we decided to rename to compose to avoid suggesting too strong an association with UI (it can return screen data, but can also be any arbitrary value).

However, this is dumb for a few reasons:

• we're still calling the type Rendering
• "render" is a much more specific verb to represent this specific concept than the super-generic "compose"
• It turns out Swift never actually got around to renaming everything anyway.

For these reasons, I propose renaming the State{ful|less}Workflow.compose method back to render.

• Slack discussion
• Kotlin issue: [kotlin] Rename compose method and concept in docs to render #265

[qrazhan]

+1 compose is really abstract and only leads to more confusion when starting out IMO.