checkdoc: Allow Lisp symbols to start a message

checkdoc: Allow Lisp symbols to start a message

* lisp/emacs-lisp/checkdoc.el (checkdoc-message-text-engine): Allow
Lisp symbols to start a message.
(checkdoc--error-bad-format-p): New helper function.

* test/lisp/emacs-lisp/checkdoc-tests.el
(checkdoc-test-error-format-is-good)
(checkdoc-test-error-format-is-bad): New helper functions.
(checkdoc-tests-error-message-bad-format-p)
(checkdoc-tests-error-message-bad-format-p/defined-symbols)
(checkdoc-tests-error-message-bad-format-p/not-capitalized):
New tests.