Set IAM Role Description

Seems like it has been added as of September 3rd, 2019. Check this part of the AWS CloudFormation docs.

Update November 2019:

The Description field is now supported in CloudFormation.

Properties: 
  AssumeRolePolicyDocument: Json
  Description: String               <--- Here
  ManagedPolicyArns: 
    - String
  MaxSessionDuration: Integer
  Path: String
  PermissionsBoundary: String
  Policies: 
    - Policy
  RoleName: String
  Tags: 
    - Tag

Tested and can confirm John Rotenstein's answer remains the best option as of 29/Mar/2019. Sometimes updates can sneak in without making the documentation, but not in this case unfortunately.

(Would have preferred to put this as a comment however the reputation requirement is a pain)

Edit: July/2019 - Still no update, however this can be done through the SDK