Update AWS::CloudFront::Function Name description with non-updable important information

[fredericbarthelet]

Description of changes:

A Cloudfront Function Name property is required and cannot be updated after resource creation. This behavior was spotted in https://github.com/getlift/lift/issues/247

Changing the name requires changing resource LogicalID to trigger old resource deletion and new resource creation. Unlike common name property whose behavior is described in https://github.com/awsdocs/aws-cloudformation-user-guide/blob/main/doc_source/aws-properties-name.md, Cloudformation cannot handle name generation for this resource type and cannot handle resource replacement.

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