Clarify OutputCache intent and limitations for classic ASP.Net.

[StephenMolloy]

This pull request adds important clarifications about the use of output caching in several ASP.NET documentation files. The main update is a warning to developers that output caching and its variation settings are intended for optimizing performance by storing alternate representations of resources, not for isolating audiences or protecting sensitive content. The warning emphasizes that any required isolation for personalized or sensitive data must be enforced in the application itself, not through cache configuration.

Documentation improvements:

• Added a warning to the ASP.NET MVC (C# and VB) output caching documentation explaining that VaryBy settings (like VaryByParam, VaryByHeader, and VaryByCustom) are for caching alternate representations, not for isolating personalized or sensitive content, and that application-level isolation is still required (improving-performance-with-output-caching-cs.md, improving-performance-with-output-caching-vb.md). [1] [2]
• Added a similar warning to the ASP.NET Web Forms caching documentation, clarifying the purpose of the @ OutputCache directive and its VaryBy settings (caching.md).
• Updated the ASP.NET 4 whitepaper to include a warning that custom output-cache providers and VaryBy combinations are not suitable for isolating sensitive or personalized responses, and that proper isolation must be handled in the application (overview.md).