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)
MFC after:	3 weeks
Differential Revision:	https://reviews.freebsd.org/D24114

lib/geom/eli/geli.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 23, 2019
+.Dd March 19, 2020
 .Dt GELI 8
 .Os
 .Sh NAME
@@ -1013,6 +1013,35 @@ geli_da1s3a_keyfile_type="da1s3a:geli_keyfile"
 geli_da1s3a_keyfile_name="/boot/keys/da1s3a.key"
 .Ed
 .Pp
+By convention, these loader variables are called
+.Sm off
+.Va geli_ No < Ar device No > Va _load .
+.Sm on
+However, the actual name prefix before
+.Va _load , _type ,
+or
+.Va _name
+does not matter.
+At boot time, the
+.Nm
+module searches through all
+.Sm off
+.No < Va prefix No > Va _type No -like
+.Sm on
+variables that have a value of
+.Sm off
+.Dq < Ar device No > :geli_keyfile .
+.Sm on
+The paths to keyfiles are then extracted from
+.Sm off
+.No < Ar prefix No > Va _name
+.Sm on
+variables.
+In the example above,
+.Ar prefix
+is
+.Dq Li geli_da1s3a_keyfile .
+.Pp
 Not only configure encryption, but also data integrity verification using
 .Nm HMAC/SHA256 .
 .Bd -literal -offset indent