Document debug-on-event

* doc/lispref/debugging.texi (Error Debugging): Mention debug-on-event.
* doc/lispref/commands.texi (Misc Events): Mention sigusr1,2 and debugging.
* etc/NEWS: Related edit.

1 files changed, 3 insertions, 1 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 1c9072d38af..67073cb1acf 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1403,7 +1403,9 @@ as well as those in the -*- line.
 
 ** keymaps can inherit from multiple parents.
 
-** `debug-on-event' lets you debug Emacs when stuck because of inhibit-quit.
++++
+** Set `debug-on-event' to make Emacs enter the debugger e.g. on receipt
+of SIGUSR1.  This can be useful when `inhibit-quit' is set.
 
 +++
 ** New reader macro ## which stands for the empty symbol.