docs(side_shift): update side shift document (#6123)

* update side shift document

Signed-off-by: kyoichi-sugahara <kyoichi.sugahara@tier4.jp>

---------

Signed-off-by: kyoichi-sugahara <kyoichi.sugahara@tier4.jp>

Author: kyoichi-sugahara

planning/behavior_path_side_shift_module/README.md

@@ -2,6 +2,29 @@
 
 (For remote control) Shift the path to left or right according to an external instruction.
 
+## Overview of the Side Shift Module Process
+
+1. Receive the required lateral offset input.
+2. Update the `requested_lateral_offset_` under the following conditions:
+   a. Verify if the last update time has elapsed.
+   b. Ensure the required lateral offset value is different from the previous one.
+3. Insert the shift points into the path if the side shift module's status is not in the SHIFTING status.
+
+Please be aware that `requested_lateral_offset_` is continuously updated with the latest values and is not queued.
+
+## Statuses of the Side Shift
+
+The side shift has three distinct statuses. Note that during the SHIFTING status, the path cannot be updated:
+
+1. BEFORE_SHIFT: Preparing for shift.
+2. SHIFTING: Currently in the process of shifting.
+3. AFTER_SHIFT: Shift completed.
+
+<figure markdown>
+  ![case1](images/side_shift_status.drawio.svg){width=1000}
+  <figcaption>side shift status</figcaption>
+</figure>
+
 ## Flowchart
 
 ```plantuml
@@ -27,7 +50,7 @@ stop
 @enduml
 ```
 
-```plantuml -->
+```plantuml
 @startuml
 skinparam monochrome true
 skinparam defaultTextAlignment center