Update documentation on -sEXIT_RUNTIME. (#25218)

Closes #25217

Update documentation to reflect the current state of affairs for the
meaning of `-sEXIT_RUNTIME`.

Author: juj

site/source/docs/tools_reference/settings_reference.rst

@@ -85,16 +85,25 @@ Default value: true
 EXIT_RUNTIME
 ============
 
-If 0, the runtime is not quit when main() completes (allowing code to
-run afterwards, for example from the browser main event loop). atexit()s
-are also not executed, and we can avoid including code for runtime shutdown,
-like flushing the stdio streams.
-Set this to 1 if you do want atexit()s or stdio streams to be flushed
-on exit.
+If 0, support for shutting down the runtime is not emitted into the build.
+This means that the program is not quit when main() completes, but execution
+will yield to the event loop of the JS environment, allowing event handlers
+to run afterwards. If 0, C++ global destructors will not be emitted into the
+build either, to save on code size. Calling exit() will throw an unwinding
+exception, but will not shut down the runtime.
+
+Set this to 1 if you do want to retain the ability to shut down the program.
+If 1, then completing main() will by default call exit(), unless a refcount
+keeps the runtime alive. Call emscripten_exit_with_live_runtime() to finish
+main() while keeping the runtime alive. Calling emscripten_force_exit() will
+shut down the runtime, invoking atexit()s, and flushing stdio streams.
 This setting is controlled automatically in STANDALONE_WASM mode:
 
 - For a command (has a main function) this is always 1
-- For a reactor (no a main function) this is always 0
+- For a reactor (no main function) this is always 0
+
+For more details, see documentation for emscripten_force_exit() and
+emscripten_exit_with_live_runtime().
 
 Default value: false
 

src/settings.js

@@ -83,17 +83,25 @@ var VERBOSE = false;
 // [link]
 var INVOKE_RUN = true;
 
-// If 0, the runtime is not quit when main() completes (allowing code to
-// run afterwards, for example from the browser main event loop). atexit()s
-// are also not executed, and we can avoid including code for runtime shutdown,
-// like flushing the stdio streams.
-// Set this to 1 if you do want atexit()s or stdio streams to be flushed
-// on exit.
+// If 0, support for shutting down the runtime is not emitted into the build.
+// This means that the program is not quit when main() completes, but execution
+// will yield to the event loop of the JS environment, allowing event handlers
+// to run afterwards. If 0, C++ global destructors will not be emitted into the
+// build either, to save on code size. Calling exit() will throw an unwinding
+// exception, but will not shut down the runtime.
+//
+// Set this to 1 if you do want to retain the ability to shut down the program.
+// If 1, then completing main() will by default call exit(), unless a refcount
+// keeps the runtime alive. Call emscripten_exit_with_live_runtime() to finish
+// main() while keeping the runtime alive. Calling emscripten_force_exit() will
+// shut down the runtime, invoking atexit()s, and flushing stdio streams.
 // This setting is controlled automatically in STANDALONE_WASM mode:
 //
 // - For a command (has a main function) this is always 1
-// - For a reactor (no a main function) this is always 0
+// - For a reactor (no main function) this is always 0
 //
+// For more details, see documentation for emscripten_force_exit() and
+// emscripten_exit_with_live_runtime().
 // [link]
 var EXIT_RUNTIME = false;