doc: Add a tip to better name mcron jobs.

* doc/guix.texi (Scheduled Job Execution): Specify the 'displayable' argument
for the 'updatedb-job' example.  Add a tip to better name jobs, referring to
the modified example.

doc/guix.texi

@@ -15919,7 +15919,8 @@ gexps to introduce job definitions that are passed to mcron
          (lambda ()
            (execl (string-append #$findutils "/bin/updatedb")
                   "updatedb"
-                  "--prunepaths=/tmp /var/tmp /gnu/store"))))
+                  "--prunepaths=/tmp /var/tmp /gnu/store"))
+         "updatedb"))
 
 (define garbage-collector-job
   ;; Collect garbage 5 minutes after midnight every day.
@@ -15948,6 +15949,14 @@ gexps to introduce job definitions that are passed to mcron
                   %base-services)))
 @end lisp
 
+@quotation Tip
+When providing the action of a job specification as a procedure, you
+should provide an explicit name for the job via the optional 3rd
+argument as done in the @code{updatedb-job} example above.  Otherwise,
+the job would appear as ``Lambda function'' in the output of
+@command{herd schedule mcron}, which is not nearly descriptive enough!
+@end quotation
+
 For more complex jobs defined in Scheme where you need control over the top
 level, for instance to introduce a @code{use-modules} form, you can move your
 code to a separate program using the @code{program-file} procedure of the