I'd love some official Apple documentation. In lieu of that, you can search man pages with `man -f`, `whatis`, or `apropos`. I also keep the following aliases in my ~/.zshrc:

  alias list-functions='functions -x4'
  alias list-function-names='functions +'
  alias list-aliases='alias'
  alias list-alias-names='alias +'
  alias list-commands='print -raC2 ${(kv)commands} | sort'
  alias list-command-names='print -roC1 ${(k)commands}'
  alias list-builtins='print -raC2 ${(kv)builtins} | sort'
  alias list-builtin-names='print -roC1 ${(k)builtins}'
  alias list-everything='whence -cm "[^_]*"'
  alias list-everything-names='whence -wm "[^_]*"'

Edit: another commenter posted what looks to be comprehensive docs/lists here: https://news.ycombinator.com/item?id=36492487