I just noticed in the "help" content which is accessed via the key 'h', it is mentionned that the '?' key stroke should display a brief list of commands. It actually prompts with a "Regexp search backward: " instead. After doing some search on internet I found that the right key that works on my version of texinfo is 'H'. I'm running slackware 14.2, the version of the GNU texinfo shipped within is 6.1.