It's true that
man nano is quite brief. However, if you have a "default" nano, you should see a small help menu at the bottom of the screen like this:
If you don't see such a menu, it is possible that you need to go into
/etc/nanorc and look for something like this:
## Don't display the helpful shortcut lists at the bottom of the screen.
# set nohelp
Uncommenting the second line causes the menu to be hidden. You may prefer to copy
/etc/nanorc to your home folder as
.nanorc and edit that copy for user-specific modifications.
You can also access nano's built-in help (and shortcuts) by pressing Ctrl+G at any time.