If using Warnings for SL messages, recommend providing WarningsNotAsErrors

[kzu]

From the feedback at https://github.com/moq/moq/discussions/1414#discussioncomment-6913884, a good point was made:

A warning would certainly be more visible, yet if WarningsNotAsErrors is provided via the package props, you can avoid the warning to ever be considered an error, even if warnings as errors is on.

Since SL provides only helpers and it's up to each package author to determine what to do with the sponsorship info, this would be mostly a documentation issue?