[Docs] Make KernelProgramCache.md doc consistent with in-memory cache implementation

[uditagarwal97]

Is your feature request related to a problem? Please describe

I see several inconsistencies between the code and the in-memory cache's working, as described in KernelProgramCache.md (https://github.com/intel/llvm/blob/sycl/sycl/doc/design/KernelProgramCache.md). For example:

1. The doc should be updated with recent changes in Cache keys
2. The doc does not explain the working/motive of Fast Kernel Cache
3. How and when we reset caches?

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Additional context

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