Clarify MemoryCacheOptions.SizeLimit doc comments

Currently, the property summary for the MemoryCacheOptions.SizeLimit property is:

```
///
/// Gets or sets the maximum size of the cache.
///
```
It would be nice if this explicitly provided the unit of measure (bytes, kilobytes, etc.). Like this:
```
///
/// Gets or sets the maximum size of the cache in kilobytes.
///
```

Start at the MemoryCacheOptions.SizeLimit property documentation and verify the unit of measure against the API definition. Update the property summary so it explicitly names the correct unit, then run the repository's documentation validation if available; done means the generated comment clearly documents that unit.

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