Improve print output options commands in backtrace-mode (bug#36566)

* lisp/emacs-lisp/backtrace.el (backtrace-view): Mention
:print-gensym in docstring.
(backtrace-mode-map): Add keyboard binding for
backtrace-toggle-print-gensym.  Add menu entries for
backtrace-toggle-print-circle and backtrace-toggle-print-gensym.
(backtrace--with-output-variables): Bind print-gensym with value
of :print-gensym found in view plist.
(backtrace-toggle-print-circle): Remove description of
implementation details from docstring.
(backtrace-toggle-print-gensym): New command.
(backtrace--toggle-feature): Add echo area message describing result
of command.

* test/lisp/emacs-lisp/backtrace-tests.el
(backtrace-tests--print-circle): New test.

* doc/lispref/debugging.texi (Backtraces): Document keyboard
binding for backtrace-toggle-print-gensym.

1 files changed, 3 insertions, 0 deletions

diff --git a/doc/lispref/debugging.texi b/doc/lispref/debugging.texi
index 12caeaf1289..71e767d0a66 100644
--- a/doc/lispref/debugging.texi
+++ b/doc/lispref/debugging.texi
@@ -457,6 +457,9 @@ Collapse the top-level Lisp form at point back to a single line.
 @item #
 Toggle @code{print-circle} for the frame at point.
 
+@item :
+Toggle @code{print-gensym} for the frame at point.
+
 @item .
 Expand all the forms abbreviated with ``...'' in the frame at point.