Documented the CMD_* env variable override

bin/pt-stalk

@@ -1843,7 +1843,49 @@ Print tool's version and exit.
 
 =head1 ENVIRONMENT
 
-This tool does not use any environment variables for configuration.
+This tool does not require any environment variables for configuration,
+although it can be influenced to work differently by through several
+variables.  Keep in mind that these are expert settings, and should not
+be used in most cases.
+
+Specifically, the variables that can be set are:
+
+=over
+
+=item CMD_GDB
+
+=item CMD_IOSTAT
+
+=item CMD_MPSTAT
+
+=item CMD_MYSQL
+
+=item CMD_MYSQLADMIN
+
+=item CMD_OPCONTROL
+
+=item CMD_OPREPORT
+
+=item CMD_PMAP
+
+=item CMD_STRACE
+
+=item CMD_SYSCTL
+
+=item CMD_TCPDUMP
+
+=item CMD_VMSTAT
+
+=back
+
+For example, during collection iostat is called with a -dx argument, but
+because you have an NFS partition, you also need the -n flag there.  Instead
+of editing the source, you can call pt-stalk as
+
+    CMD_IOSTAT="iostat -n" pt-stalk ...
+
+which will do exactly what you need.  Combined with the plugin hooks, this
+gives you a fine-grained control of what the tool does.
 
 =head1 SYSTEM REQUIREMENTS