zdev/dracut/module-setup.sh: add comments for dracut called functions

Add a short explaining comment to those functions, which are just callbacks
invoked by the dracut core infrastructure. So nobody wonders how these
functions get called, especially if there are no local callers here in the
code.

Cf. dracut commit
https://github.com/dracutdevs/dracut/commit/8bcfd683bd0c
("*/module-setup.sh: add comments for dracut called functions")

For more information about the dracut module callback functions:
https://github.com/dracutdevs/dracut/blob/master/man/dracut.modules.7.asc#writing-a-module
https://github.com/dracutdevs/dracut/blob/master/docs/HACKING.md#writing-modules

Github-ID: https://github.com/ibm-s390-linux/s390-tools/pull/158
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Acked-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Acked-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

zdev/dracut/95zdev/module-setup.sh

@@ -13,6 +13,7 @@
 #   a hook is installed to parse rd.zdev= kernel parameters.
 #
 
+# called by dracut
 check() {
     local _arch
     _arch=$(uname -m)
@@ -33,10 +34,12 @@ check() {
     return 0
 }
 
+# called by dracut
 depends() {
     return 0
 }
 
+# called by dracut
 installkernel() {
     # Add modules for all device types supported by chzdev (required for
     # auto-configuration)
@@ -44,6 +47,7 @@ installkernel() {
 	     dasd_diag_mod zfcp
 }
 
+# called by dracut
 install() {
     local _tempfile