Improve documentation of debugging Lisp syntax error

* doc/lispref/debugging.texi (Syntax Errors, Excess Open)
(Excess Close): Name the commands invoked by the key
sequences.  Add cross-references to appropriate sections of
the Emacs manual.  (Bug#21385)
1 job for master in 76 minutes and 46 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #2860
test-all

01:16:46

 
Name Stage Failure
failed
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1