Document how the Godot binary loads projects, and how to load external Godot projects from a Godot launcher with OS.execute

**Godot version:**

v3.2.stable.official

**OS/device including version:**

N/A

**Issue description:**

Running "app1" in the Godot editor. App1 calls OS.execute to launch "app2.exe" which is another Godot app created using Windows Desktop export template with debug turned off. Instead of running app2.exe, a second instance of app1 ran.

When I export app1 using the Windows Desktop export template and run the app1.exe, OS.execute launches the correct app (app2.exe)

**Steps to reproduce:**
1) Create a simple project and export to "_path_/app2.exe" using Windows Desktop export template with debug turned off. Verify this app runs correctly when launched manually.

2) Create a second simple projected called "app1" with a button that when clicked calls OS.execute("_path_/app2.exe",[],false).

3) Run app1 in the editor, click the button. A second instance of app1 appears.

4) Export app1 to "_path_/app1.exe" using the Windows Desktop export template with debug turned off.

5) Launch app1.exe and click the button. app2.exe is executed correctly.

**Minimal reproduction project:**