Documentation comes installed with MINIX. Much helpful information is in MINIX's manual pages (or “manpages”), which can be read with man or online.
For example, to learn about the Ash shell, type:
man ash
Sometimes you'll see manual pages mentioned like this: shutdown(8). This refers to the shutdown page in section 8 of the manual.
You can read it like this:
man 8 shutdown
To find installed manual pages, please look in /usr/man and /usr/pkg/man.
Finally, you can read the manual page about man itself:
man man