ProcessorCount doc should mention DOTNET_PROCESSOR_COUNT environment variable

### Type of issue

Missing information

### Description

According to https://learn.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/6.0/environment-processorcount-on-windows the environment variable `DOTNET_PROCESSOR_COUNT` ultimately sets the `ProcessorCount` property. This fact should be mentioned in the API doc.

### Page URL

https://learn.microsoft.com/en-us/dotnet/api/system.environment.processorcount?view=net-8.0

### Content source URL

https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System/Environment.xml

### Document Version Independent Id

2221a3a7-144c-eade-0ecd-750fa77bb662

### Article author

@dotnet-bot