doc/releases: Note "main" return type change for 3.4

Add a paragraph explaining the change from void main(void) to int
main(void) and the requirement for main functions to return zero.

Signed-off-by: Keith Packard <keithp@keithp.com>

doc/releases/release-notes-3.4.rst

@@ -129,6 +129,17 @@ Changes in this release
   * Touchscreen drivers converted to use the input APIs can use the
     :dtcompatible:`zephyr,kscan-input` driver to maintain Kscan compatilibity.
 
+* The declaration of :c:func:`main` has been changed from ``void
+  main(void)`` to ``int main(void)``. The main function is required to
+  return the value zero. All other return values are reserved. This aligns
+  Zephyr with the C and C++ language specification requirements for
+  "hosted" environments, avoiding compiler warnings and errors. These
+  compiler messages are generated when applications are built in "hosted"
+  mode (which means without the ``-ffreestanding`` compiler flag). As the
+  ``-ffreestanding`` flag is currently enabled unless the application is
+  using picolibc, only applications using picolibc will be affected by this
+  change at this time.
+
 Removed APIs in this release
 ============================
 

samples/hello_world/src/main.c

@@ -6,8 +6,8 @@
 
 #include <zephyr/kernel.h>
 
-int main(void)
+void main(void)
 {
 	printk("Hello World! %s\n", CONFIG_BOARD);
-	return 0;
+	return;
 }