Cygwin: document new winjitdebug option

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/release/3.5.1

@@ -1,8 +1,14 @@
-Bug Fixes
----------
+Fixes:
+------
 
 - Fix exit code for non-cygwin process running in console. The bug
   was introduced in 3.5.0.
 
 - Make the interface names handled by if_nametoindex() and if_indextoname()
   consistent with that of if_nameindex().
+
+- Revert error handling for non-Cygwin child processes to use Cygwin's
+  error mode by default.  Resetting the error mode to the OS default
+  error mode is now possible by using the new CYGWIN environment variable
+  option "winjitdebug".
+  Addresses: https://cygwin.com/pipermail/cygwin/2024-February/255305.html

winsup/doc/cygwinenv.xml

@@ -93,6 +93,16 @@ line (truncated to ~32K) will be passed on any processes that it creates
 in addition to the normal UNIX argv list.  Defaults to not set.</para>
 </listitem>
 
+<listitem>
+<para><envar>(no)winjitdebug</envar> - if set, non-Cygwin executables
+started from a Cygwin process will have their error mode reset to the
+system default.  That means, if they don't call SetErrorMode() explicitl,
+the OS will display error dialog boxes in the GUI if some fatal error
+occurs.  Defaults to not set, so fatal errors in a non-Cygwin child
+process will just result in an error code be propagated back to the
+parent process.</para>
+</listitem>
+
 <listitem>
 <para><envar>winsymlinks:{lnk,native,nativestrict,sys}</envar></para>