bug#20754: checkdoc-current-buffer prints to stdout?!

[Paul Eggert <eggert@cs.ucla.edu>]

[The following bug report was sent by Sebastian Wiesner to emacs-devel 
<http://lists.gnu.org/archive/html/emacs-devel/2015-06/msg00095.html>.]

Hello,

while debugging unit test failures in Flycheck, I accidentally discovered
checkdoc-current-buffer apparently prints messages to standard output in
Emacs 25.1 in noninteractive mode.

Would you please revert this change?

I can understand the motivation for this “feature”, but it's generally a
bad idea for an interactive function to print text to standard output
depending on an opaque global environment.  It also makes backwards
compatibility harder than it needs to be:  As things stand I have no choice
but to check the Emacs version to print Checkdoc Errors in a consistent way
across Emacs major versions, and that is somewhat ugly.

The purpose of this feature would also be served by a different entry
point—we have `ert-run-tests-interactively` and `ert-run-tests-batch`,
too.  If that is impossible please at least expose an option to turn this
feature off.

On a related topic, I'd have fixed this issue much faster, had this change
been mentioned in the NEWS.  Would you please be more strict about updating
NEWS for breaking changes such as this?

Sincerely,
Sebastian Wiesner