MFC 359125:

Document geli(8) loader variables conventions

The geli(8) manual page has an example for preloading keyfiles during boot.
There is no detail though on how the lookup of these variables actually
works.

Let's document that the name of a device does not have to be a part
of the variable.

PR: 243261
Submitted by: johannes@jo-t.de
Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D24114