APIs which contain oblivious nullability should indicate that in Quick Info

Recently we made a change to give a heads up when interacting with nullable-oblivious variables:

It would be useful if this information were also conveyed when looking at a signature that contains oblivious nullability:

No files or tests are named. Start by reproducing the two Quick Info cases shown in the issue and trace the existing nullable-oblivious variable indication; done means signatures containing oblivious nullability visibly convey that information in Quick Info.

Written by the indexing model from the issue text.