Improve documentation of rd.memdebug

Explain more about rd.memdebug verbose level in dracut.cmdline.7.asc.

Signed-off-by: Dave Young <dyoung@redhat.com>

dracut.cmdline.7.asc

@@ -144,8 +144,12 @@ It should be attached to any report about dracut problems.
     If "quiet" is set, it also logs to the console.
 
 **rd.memdebug=[0-3]**::
-    print memory usage debug info, set the verbose level from 1 to 3
-    print nothing when set rd.memdebug=0
+    Print memory usage info at various points, set the verbose level from 0 to 3
+    Higher level means more debugging output:
+    0 - no output
+    1 - partial /proc/meminfo
+    2 - /proc/meminfo
+    3 - /proc/meminfo + /proc/slabinfo
 
 **rd.break**::
     drop to a shell at the end