docs: document headerBackTestID option in stack (#1234)

Co-authored-by: Satyajit Sahoo <satyajit.happy@gmail.com>

Author: tranquanghuy0801

versioned_docs/version-6.x/stack-navigator.md

@@ -370,6 +370,10 @@ Title string used by the back button when `headerBackTitle` doesn't fit on the s
 
 Style object for the back title.
 
+#### `headerBackTestID`
+
+Test ID for the back button for testing purposes.
+
 ### Events
 
 The navigator can [emit events](navigation-events.md) on certain actions. Supported events are:

versioned_docs/version-7.x/stack-navigator.md

@@ -364,6 +364,10 @@ Defaults to `default` on iOS, and `minimal` on Android.
 
 Style object for the back title.
 
+#### `headerBackTestID`
+
+Test ID for the back button for testing purposes.
+
 ### Events
 
 The navigator can [emit events](navigation-events.md) on certain actions. Supported events are:

versioned_docs/version-8.x/stack-navigator.md

@@ -364,6 +364,10 @@ Defaults to `default` on iOS, and `minimal` on Android.
 
 Style object for the back title.
 
+#### `headerBackTestID`
+
+Test ID for the back button for testing purposes.
+
 ### Events
 
 The navigator can [emit events](navigation-events.md) on certain actions. Supported events are: