By default, the zsh help command does not show help for builtins. Add it:

autoload -Uz run-help
unalias run-help &>/dev/null
alias help='PAGER="less -FX" run-help'

Example usage:

% help fg
fg [ job ... ]
job ...
       Bring each specified job in turn to the foreground.  If  no  job
       is specified, resume the current job.