doc: add baseline mode in l3fwd-power guide

Updating PMD Power Management Mode section of the l3fwd-power
sample app to add baseline option as this was missing from the
original commit.

Fixes: a9ea607 ("examples/l3fwd-power: add baseline PMD management mode")
Cc: stable@dpdk.org

Signed-off-by: Karen Kelly <karen.kelly@intel.com>

doc/guides/sample_app_ug/l3_forward_power_man.rst

@@ -280,6 +280,9 @@ will use automatic PMD power management.
 This mode is limited to one queue per core,
 and has three available power management schemes:
 
+``baseline``
+  This mode will not enable any power saving features.
+
 ``monitor``
   This will use ``rte_power_monitor()`` function to enter
   a power-optimized state (subject to platform support).