Is there a unix shell on this planet that does not have the -c option? :-)

But yes, there's no clear path leading to busybox shell docs, until one starts digging a bit harder, which I did a while ago:

The busybox shell originates from the almquist shell (ash). ash became dash when is was ported to debian, and later became the default /bin/sh on ubuntu. And ash was also included into busybox.

So when you want to know something about the busybox shell, the closest match is a dash man page.