Begin: WIP consistent Begin/End behavior.

docs/FAQ.md

@@ -429,8 +429,10 @@ provide similar or better string helpers.
 ### Q: How can I display custom shapes? (using low-level ImDrawList API)
 
 - You can use the low-level `ImDrawList` api to render shapes within a window.
+
 ```cpp
-ImGui::Begin("My shapes");
+if (!ImGui::Begin("My shapes"))
+    return;
 
 ImDrawList* draw_list = ImGui::GetWindowDrawList();
 

docs/README.md

@@ -52,7 +52,8 @@ Result:
 Code:
 ```cpp
 // Create a window called "My First Tool", with a menu bar.
-ImGui::Begin("My First Tool", &my_tool_active, ImGuiWindowFlags_MenuBar);
+if (!ImGui::Begin("My First Tool", &my_tool_active, ImGuiWindowFlags_MenuBar))
+    return;
 if (ImGui::BeginMenuBar())
 {
     if (ImGui::BeginMenu("File"))