[docs] Add thread/process modes in Configure Functions runtime (#4828)

* add thread/process modes

* update the differences between thread and process modes

* update

* update

(cherry picked from commit 8c163eb2)

site2/docs/functions-overview.md

@@ -509,4 +509,4 @@ $ bin/pulsar-admin functions create \
 Pulsar Functions that use [Pulsar Functions SDK](#the-pulsar-functions-sdk) can publish metrics to Pulsar. For more information, see [Metrics for Pulsar Functions](functions-metrics.md).
 
 ## State storage
-Pulsar Functions use [Apache BookKeeper](https://bookkeeper.apache.org) as a state storage interface. Pulsar installation, including the local standalone installation, includes deployment of BookKeeper bookies.
\ No newline at end of file
+Pulsar Functions use [Apache BookKeeper](https://bookkeeper.apache.org) as a state storage interface. Pulsar installation, including the local standalone installation, includes deployment of BookKeeper bookies.

site2/docs/functions-runtime.md

@@ -10,6 +10,10 @@ Currently, Pulsar Functions support the following three methods to run functions
 - *Process*: Invoke functions in processes forked by Functions Worker.
 - *Kubernetes*: Submit functions as Kubernetes StatefulSets by Functions Worker.
 
+The differences of the thread and process modes are:   
+- Thread mode: when a function runs in thread mode, it runs on the same Java virtual machine (JVM) with Functions worker.   
+- Process mode: when a function runs in process mode, it runs on the same machine that Functions worker runs.
+
 ## Configure thread runtime
 
 *Thread* runtime is easy to configure. In most cases, you don't need to configure anything. You can customize the thread group name with the following settings: