Update api.md

Author: sumerc

doc/api.md

@@ -28,7 +28,7 @@ All results stay in memory unless application (all threads including th
 #### `get_func_stats(filter=None)`
 
 Returns the function stats as a [`YFuncStat`](#yfuncstat) object.
-filter parameter can be used to filter on YFuncStat attributes. You can use multiple filters at once in a single call and only those results are returned. 
+filter parameter can be used to filter on YFuncStat attributes. You can use multiple filters at once in a single call and only those results are returned. If no filter is defined, all function stats are aggregated(function stats are held per-thread under the hood) and returned. 
 
 One of the interesting features of this filter function is that you can get per-thread function call statistics only by providing the ctx_id of the thread you want to get results. Under the hood, yappi already holds the function stats by per-thread and upon request, it aggregates this data, when you provide a filter, it simply returns only that per-thread stats.