Fix invocation with --debug-init and changes to debug-ignored-errors

Fix invocation with --debug-init and changes to debug-ignored-errors

* src/eval.c (syms_of_eval) <debug-ignored-errors>: Mention in the
doc string the caveat with removing errors from the standard value
while invoking Emacs with --debug-init.

* lisp/startup.el (startup--load-user-init-file): If the user's
init files add to the value of 'debug-ignored-errors', make sure
the additions are preserved after restoring the value we reset
during loading the init file, if Emacs was invoked with
"--debug-init".  (Bug#65267)