Add docs remarks for IsRealNumber handling of infinities

[FilipToth]

Summary

Added documentation coverage for IsRealNumber methods' handling of infinities, which may seem unintuitive to some developers. For further details, refer to the linked issue.

A separate PR will be submitted into dotnet/runtime to update inline documentation comments once this change is approved, in order to avoid simultaneous changes across two repos.

Fixes dotnet/runtime#103930

[learn-build-service-prod]

Learn Build status updates of commit e07db1b:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Numerics/Complex.xml	✅Succeeded	View
xml/System.Numerics/INumberBase`1.xml	✅Succeeded	View
xml/System.Runtime.InteropServices/NFloat.xml	✅Succeeded	View
xml/System/Double.xml	✅Succeeded	View
xml/System/Half.xml	✅Succeeded	View
xml/System/Single.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[AaronRobinsonMSFT]

@tannergooding Are you okay with this update?

[learn-build-service-prod]

Learn Build status updates of commit de098e3:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Numerics/INumberBase`1.xml	✅Succeeded	View
xml/System.Runtime.InteropServices/NFloat.xml	✅Succeeded	View
xml/System/Double.xml	✅Succeeded	View
xml/System/Half.xml	✅Succeeded	View
xml/System/Single.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel