docs: Improve docs on MDADM_CONFIG

doc/genkernel.8.txt

@@ -251,6 +251,8 @@ INITIALIZATION
 
 *--mdadm-config*=<file>::
     Use <file> as configfile for MDADM.
+    By default the ramdisk will be built *without* an mdadm.conf and
+    will auto-detect arrays during boot-up.
 
 *--dmraid*::
     Include DMRAID support.

genkernel.conf

@@ -73,11 +73,11 @@ USECOLOR="yes"
 # Includes mdadm/mdmon binaries in initramfs.
 # Without sys-fs/mdadm[static] installed, this will build a static mdadm.
 # MDADM="no"
+
 # Specify a custom mdadm.conf.
-# If not set the ramdisk will be built without a mdadm.conf and will auto-detect
-# arrays during bootup.
-# This should generally not be needed.
-# MDADM_CONFIG=""
+# By default the ramdisk will be built *without* an mdadm.conf and will auto-detect
+# arrays during bootup.  Usually, this should not be needed.
+# MDADM_CONFIG="/etc/mdadm.conf"
 
 # Add Multipath support.
 # MULTIPATH="no"