The 'help' command is exactly what you're looking for -- e.g., 'help disown' will describe the 'disown' command. Definitely beats searching the entire bash man page for the command you want (which is what I did for far too long, until I found out about 'help').

Yeah, but it often isn't as long as a real man page. Plus for some reason zsh doesn't have it!

There's always google you know :D. The point is to know what to look for:

$ help "*"

will print out all shell built ins (list help for them). And if the short description is not enough, go google for each.