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)

(cherry picked from commit faafd467)
1 job for emacs-26 in 46 minutes and 56 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #2859
test

00:46:56

 
Name Stage Failure
failed
test Test
Makefile:285: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:255: recipe for target 'check' failed
make[1]: *** [check] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:951: recipe for target 'check' failed
make: *** [check] Error 2
ERROR: Job failed: exit code 1