update doc of inc_pluginfunctions

inc_pluginfunctions.md

@@ -88,10 +88,14 @@ Execute a command and repeat max. MAXTRIES times if it fails.
 
 Add performance data. Their output will be written with ph.exit. So you are free to add perfomance data anywhere within your check script.
 
-**ph.perfdeltaspeed** [VARNAME] [VALUE]
+**ph.perfdeltaspeed** [VARNAME] [VALUE] [[unit] [isfloat]]
 
 For increasing system counters: get changerate per second since last check.
 
+Unit value can be
+* s or sec - for seconds
+* m or min - for minutes
+
 Example:
 
     # speed in byte per sec based on last stored value and its age