Improve documentation on `DefaultKeySet`

### Type of issue

Missing information

### Description

The value `DefaultKeySet` is not clearly documented. I would suggest: Private keys are stored in the current user store unless the certificate specifies that the keys should go in the local computer store.

### Page URL

https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography.x509certificates.x509keystorageflags?view=net-9.0

### Content source URL

https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Security.Cryptography.X509Certificates/X509KeyStorageFlags.xml

### Document Version Independent Id

3f362403-2677-9750-42ee-c032e62bc6d6

### Platform Id

2ff394a2-d906-529c-0eff-ba9bc4c5eca6

### Article author

@dotnet-bot