Document the relation between $command and $foo_program.

PR:		docs/68453
MFC after:	3 days

share/man/man8/rc.subr.8

@@ -449,6 +449,8 @@ Full path to the command.
 Not required if
 .Ar argument Ns Va _cmd
 is defined for each supported keyword.
+Can be overridden by
+.Va ${name}_program .
 .It Va command_args
 Optional arguments and/or shell directives for
 .Va command .
@@ -537,6 +539,19 @@ as.
 Only supported after
 .Pa /usr
 is mounted.
+.It Va ${name}_program
+Full path to the command.
+Overrides
+.Va command
+if both are set, but has no effect if
+.Va command
+is unset.
+As a rule,
+.Va command
+should be set in the script while
+.Va ${name}_program
+should be set in
+.Xr rc.conf 5 .
 .It Va ${name}_user
 User to run
 .Va command