docs: Document change in bahavior of --mdadm/MDADM="yes"

doc/genkernel.8.txt

@@ -254,7 +254,8 @@ INITIALIZATION
     exist.
 
 *--mdadm*::
-    Copy '/etc/mdadm.conf' to initramfs.
+    Copy '/etc/mdadm.conf' and mdadm/mdmon binaries to initramfs.
+    Without sys-fs/mdadm[static] installed, this will compile mdadm for you.
 
 *--dmraid*::
     Add DMRAID support.

genkernel.conf

@@ -70,7 +70,8 @@ USECOLOR="yes"
 # If included, busybox is rebuilt if the cached copy is out of date.
 # BUSYBOX="yes"
 
-# Copy /etc/mdadm.conf to initramfs.
+# Copy /etc/mdadm.conf, mdadm/mdmon binaries to initramfs.
+# Without sys-fs/mdadm[static] installed, this will build a static mdadm.
 # MDADM="no"
 
 # Add Multipath support.