Correct some more custom type specs

* lisp/winner.el (winner-boring-buffers-regexp):
* lisp/progmodes/js.el (js-jsx-indent-level):
* lisp/image-dired.el (image-dired-external-viewer):
* lisp/gnus/nnir.el (nnir-notmuch-filter-group-names-function):
Correct custom type specification.

* lisp/textmodes/bibtex.el (bibtex-string-file-path):
(bibtex-file-path): Correct custom type specification and document
source of initial value.

lisp/gnus/nnir.el

@@ -535,7 +535,7 @@ like so:
    (lambda (g) (replace-regexp-in-string \"\\\\.\" \"/\" g)))"
   :version "27.1"
   :type '(choice function
-		 nil))
+		 (const :tag "No" nil)))
 
 ;;; Developer Extension Variable:
 

lisp/image-dired.el

@@ -538,7 +538,9 @@ with the comment."
   "Name of external viewer.
 Including parameters.  Used when displaying original image from
 `image-dired-thumbnail-mode'."
-  :type 'string
+  :version "27.1"
+  :type '(choice string
+                 (const :tag "Not Set" nil))
   :group 'image-dired)
 
 (defcustom image-dired-main-image-directory "~/pics/"

lisp/progmodes/js.el

@@ -650,7 +650,8 @@ indentation looks like this (different):
       </element>
   )"
   :version "27.1"
-  :type 'integer
+  :type '(choice integer
+                 (const :tag "Not Set" nil))
   :safe (lambda (x) (or (null x) (integerp x)))
   :group 'js)
 ;; This is how indentation behaved out-of-the-box until Emacs 27.  JSX

lisp/textmodes/bibtex.el

@@ -933,9 +933,12 @@ to the directories specified in `bibtex-string-file-path'."
   :type '(repeat file))
 
 (defcustom bibtex-string-file-path (getenv "BIBINPUTS")
-  "Colon-separated list of paths to search for `bibtex-string-files'."
+  "Colon-separated list of paths to search for `bibtex-string-files'.
+Initialized from the BIBINPUTS environment variable."
   :group 'bibtex
-  :type 'string)
+  :version "27.1"
+  :type '(choice string
+                 (const :tag "Not Set" nil)))
 
 (defcustom bibtex-files nil
   "List of BibTeX files that are searched for entry keys.
@@ -949,9 +952,12 @@ See also `bibtex-search-entry-globally'."
                          directory file)))
 
 (defcustom bibtex-file-path (getenv "BIBINPUTS")
-  "Colon separated list of paths to search for `bibtex-files'."
+  "Colon separated list of paths to search for `bibtex-files'.
+Initialized from the BIBINPUTS environment variable."
   :group 'bibtex
-  :type 'string)
+  :version "27.1"
+  :type '(choice string
+                 (const :tag "Not Set" nil)))
 
 (defcustom bibtex-search-entry-globally nil
   "If non-nil, interactive calls of `bibtex-search-entry' search globally.

lisp/winner.el

@@ -61,7 +61,8 @@ You may want to include buffer names such as *Help*, *Apropos*,
 
 (defcustom winner-boring-buffers-regexp nil
   "`winner-undo' will not restore windows with buffers matching this regexp."
-  :type 'string
+  :type '(choice (regexp :tag "Regexp")
+                 (const :tag "Not Set" nil))
   :version "27.1")