safeRenderCompletion documentation

[Steffan-Ennis]

Prerequisites

• [x ] I have read the documentation;
• [x ] In the case of a bug report, I understand that providing a SSCCE example is tremendously useful to the maintainers.

Description

Should document the purpose of the safeRenderCompletion prop

When onChange has been invoked it should invoke props.onChange with the formData taken from it's arguments. Instead if props.safeRenderCompletion !== true, props.onChange is invoked outside of the component.setState callback.

This creates issues because the state change may not have been resolved before props.onChange has been invoked.

[epicfaace]

Merging this into #1197.