Make checkdoc-verb-check-experimental-flag default to nil

This user option leads to a very large amount of false positives in
practice.  In my personal experience, almost all occurrences that are
flagged are false positives.  Flipping the default to something less
intrusive should hopefully encourage more use of checkdoc.

* lisp/emacs-lisp/checkdoc.el
(checkdoc-verb-check-experimental-flag): Switch the default to nil.

etc/NEWS

@@ -95,6 +95,11 @@ backend.
 This affects calls to 'warn', 'lwarn', 'display-warning', and
 'message-box'.
 
+---
+*** The default of 'checkdoc-verb-check-experimental-flag' is now nil.
+In most cases, having it enabled leads to a large amount of false
+positives.
+
 
 * New Modes and Packages in Emacs 31.1
 

lisp/emacs-lisp/checkdoc.el

@@ -308,11 +308,12 @@ problem discovered.  This is useful for adding additional checks.")
 (defvar checkdoc-diagnostic-buffer "*Style Warnings*"
   "Name of warning message buffer.")
 
-(defcustom checkdoc-verb-check-experimental-flag t
+(defcustom checkdoc-verb-check-experimental-flag nil
   "Non-nil means to attempt to check the voice of the doc string.
 This check keys off some words which are commonly misused.  See the
 variable `checkdoc-common-verbs-wrong-voice' if you wish to add your own."
-  :type 'boolean)
+  :type 'boolean
+  :version "31.1")
 ;;;###autoload(put 'checkdoc-verb-check-experimental-flag 'safe-local-variable #'booleanp)
 
 (defvar checkdoc-generate-compile-warnings-flag nil
@@ -346,6 +347,7 @@ See Info node `(elisp) Documentation Tips' for background."
 ;; (setq checkdoc--argument-missing-flag nil)      ; optional
 ;; (setq checkdoc--disambiguate-symbol-flag nil)   ; optional
 ;; (setq checkdoc--interactive-docstring-flag nil) ; optional
+;; (setq checkdoc-permit-comma-termination-flag t) ; optional
 ;; (setq checkdoc-verb-check-experimental-flag nil)
 ;; Then use `M-x find-dired' ("-name '*.el'") and `M-x checkdoc-dired'