Update docs for SRC.MemoryCache configuration element

[StephenMolloy]

This pull request updates the documentation for cache settings in the .NET Framework, providing clearer descriptions for certain attributes in the memorycache-element-cache-settings.md file. This mirrors the updates we just made for the MemoryCache API in https://github.com/dotnet/dotnet-api-docs/pull/11375

Documentation updates:

• CacheMemoryLimitMegabytes: Clarified that this setting is only effective on the .NET Framework.
• PhysicalMemoryLimitPercentage: Updated the description to specify that the percentage refers to total physical memory usage by all processes, at which point the cache begins to evict entries.

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docs/framework/configure-apps/file-schema/runtime/memorycache-element-cache-settings.md	docs/framework/configure-apps/file-schema/runtime/memorycache-element-cache-settings