Jean Shepherd fans who think they've spotted his voice on quite a few radio commercials lately are absolutely right. The humorist has indeed been "discovered" by advertising land and has a whole bunch of TV spots coming up as well. "When you're hot, you're hot," he explained yesterday. Already in the can are on camera spots for Greyhound (Grey Advertising), Sylvania TV (Doyle Dane Bernbach), Kodak (J. Walter Thompson) and Alka Seltzer (Wells, Rich, Greene). Mr. Shepherd doesn't think the agencies were attracted to him by his WOR radio show ("They only listen to other people's commercials") but might have caught him on his Public Broadcasting System television program. He's no stranger to TV commercials, but in a lot of his past assignments he's been supplying just the voice, not the face, as with this last season's Rheingold commercials for the Met games. Asked about his sudden new popularity with the world of commerce, he responded in Jean Shepherd style with, "sheer talent."