MINOR: pools: preset the allocation failure rate to 1% with -dMfail

Using -dMfail alone does nothing unless tune.fail-alloc is set, which
renders it pretty useless as-is, and is not intuitive. Let's change
this so that the filure rate is preset to 1% when the option is set on
the command line. This allows to inject failures without having to edit
the configuration.
diff --git a/doc/management.txt b/doc/management.txt
index 94451b5..96bd30c 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -290,7 +290,8 @@
       - fail / no-fail:
         This enables randomly failing memory allocations, in conjunction with
         the global "tune.fail-alloc" setting. This is used to detect missing
-        error checks in the code.
+        error checks in the code. Setting the option presets the ratio to 1%
+        failure rate.
 
       - no-merge / merge:
         By default, pools of very similar sizes are merged, resulting in more