Make sure that declarative defvars do not set variable value

* lisp/ob-tangle.el (org-id-link-to-org-use-id):
* lisp/org-agenda.el (org-agenda-overriding-header): Do not try set
value of a variable defined elsewhere.  This may interfere with custom
interface.  See
https://list.orgmode.org/orgmode/871r19ryxb.fsf@localhost/

(org-agenda-restrict):
* lisp/ol-bibtex.el (org-bibtex-description): Remove incorrect
comments.

lisp/ob-tangle.el

@@ -43,7 +43,7 @@
 (declare-function org-in-commented-heading-p "org" (&optional no-inheritance))
 (declare-function org-in-archived-heading-p "org" (&optional no-inheritance))
 (declare-function outline-previous-heading "outline" ())
-(defvar org-id-link-to-org-use-id nil) ; Dynamically scoped
+(defvar org-id-link-to-org-use-id) ; Dynamically scoped
 
 (defcustom org-babel-tangle-lang-exts
   '(("emacs-lisp" . "el")

lisp/ol-bibtex.el

@@ -115,7 +115,7 @@
 
 (defvar org-agenda-overriding-header)
 (defvar org-agenda-search-view-always-boolean)
-(defvar org-bibtex-description nil) ; dynamically scoped from org.el
+(defvar org-bibtex-description nil)
 (defvar org-id-locations)
 (defvar org-property-end-re)
 (defvar org-special-properties)

lisp/org-agenda.el

@@ -99,8 +99,8 @@
 
 ;; Defined somewhere in this file, but used before definition.
 (defvar org-agenda-buffer-name "*Org Agenda*")
-(defvar org-agenda-overriding-header nil)
 (defvar org-agenda-title-append nil)
+(defvar org-agenda-overriding-header)
 ;; (with-no-warnings (defvar entry)) ;; unprefixed, from calendar.el
 ;; (with-no-warnings (defvar date))  ;; unprefixed, from calendar.el
 (defvar original-date) ; dynamically scoped, calendar.el does scope this
@@ -2158,7 +2158,7 @@ string that it returns."
 (org-remap org-agenda-mode-map 'move-end-of-line 'org-agenda-end-of-line)
 
 (defvar org-agenda-menu) ; defined later in this file.
-(defvar org-agenda-restrict nil) ; defined later in this file.
+(defvar org-agenda-restrict nil)
 (defvar org-agenda-follow-mode nil)
 (defvar org-agenda-entry-text-mode nil)
 (defvar org-agenda-clockreport-mode nil)