Should I inform the editor about a generalized result of a result in a paper under review?

I myself would do (2). It adds information, enhances interest and acceptability, and is basic openness. The goal here is to enlighten and advance our understanding of the subject matter. Good luck! You should probably also prepare a careful revision for the referee's consideration.

Theorem B is the result of additional work of yours after submission of your paper of A. I would consider writing a follow-up paper later about that work. The additional advantage is that it adds a publication in your name. If my paper A as submitted would be refused, then I would write a paper A+B and submit it, probably to another journal. But anyway, before trying to add additional work B in paper A I would thoroughly check the journal's guidelines for submission to see whether that would fit or not.