kernel: Document TARGET_KERNEL_RECOVERY_CONFIG usage

Change-Id: Idefd38dae5e8beac7d7de797f3f792fea25dacef
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index ef6ae8f..9b8c873 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -22,8 +22,9 @@
 # These config vars are usually set in BoardConfig.mk:
 #
 #   TARGET_KERNEL_CONFIG               = List of kernel defconfigs, first one being the base one,
-#                                          while all the others are fragments that will be merged.
+#                                          while all the others are fragments that will be merged
 #                                          to main one in .config.
+#   TARGET_KERNEL_RECOVERY_CONFIG      = Same as above, but applicable to recovery kernel instead.
 #   TARGET_KERNEL_VARIANT_CONFIG       = Variant defconfig, optional
 #   TARGET_KERNEL_SELINUX_CONFIG       = SELinux defconfig, optional
 #