Fn::If inside Fn::Contains in Assert block of Rule fails template validation

### Name of the resource

Other

### Resource Name

CloudFormation Rules Block

### Issue Description

Rule-specific intrinsic function "Fn::If" cannot be used under Rules block of CloudFormation template.
Feature Request Description:
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/rules-section-structure.html#rules-specific-intrinsic-section-structure .

This document specifies that the "Fn::If" can be used inside the Rule Block and it states that, you can nest functions, but the final result of a rule condition or assertion must be either true or false.

But this does not seem to be possible in this case as it fails at template validation step.

### Expected Behavior

Intrinsic function in the Rules block should get evaluated.

### Observed Behavior

Template Fails Validation

### Test Cases

Sample Template:

{
"AWSTemplateFormatVersion": "2010-09-09",
"Parameters": {
"Environment": {
"Type": "String"
},
"Cluster":{
"Type": "String"
}
},
"Rules": {
"FargateCluster": {
"Assertions": [
{
"Assert": {
"Fn::Contains": [
[
"fargate-production",
"fargate-staging"
],
{
"Fn::If": [
"IsProduction",
"fargate-production",
"fargate-staging"
]
}
]
},
"AssertDescription": "For Fargate Spot services, Cluster must be one of fargate-production, fargate-staging"
}
]
}
},
"Resources": {
"S3Bucket": {
"Type": "AWS::S3::Bucket",
"Properties": {
"BucketName": "somesamplebucket"
}
}
}
}

Error Message: Template format error: Following functions are not supported in the Rules block of the template: [Fn::If]

This contradicts what is said in the following link:
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/rules-section-structure.html

"You can use the following rule-specific intrinsic functions to define rule conditions and assertions: "

### Other Details

N/A

Start with the sample template in the issue and the linked CloudFormation Rules documentation. Reproduce the validation error showing that Fn::If is rejected inside Fn::Contains in an Assert block, then determine whether the documented behavior or the service validation should change; done means the discrepancy is resolved and the sample validates as expected.

Written by the indexing model from the issue text.