In ReSharper 8.0 (latest EAP) without StyleCop, go to ReSharper | Options -> Code Editing | XML Doc Comments | Formatting Style, and in section Tag content find two options named Indent inside element.... Set them to No indent at all to avoid indentation. I don’t know if that would work with StyleCop.
In JetBrains Rider, the same setting can be found at File | Settings (Options on Mac) -> Editor -> Code Style -> C# | XML documentation