Document the use of -xrm to start EMacs in synchronous mode.

etc/DEBUG

@@ -249,6 +249,15 @@ mode, where each Xlib call checks for errors before it returns.  This
 mode is much slower, but when you get an error, you will see exactly
 which call really caused the error.
 
+You can start Emacs in a synchronous mode by invoking it with the -xrm
+option, like this:
+
+    emacs -rm "emacs.synchronous: true"
+
+Setting a breakpoint in the function `x_error_quitter' and looking at
+the backtrace when Emacs stops inside that function will show what
+code causes the X protocol errors.
+
 ** If the symptom of the bug is that Emacs fails to respond
 
 Don't assume Emacs is `hung'--it may instead be in an infinite loop.