Don't quote nil and t in doc strings

This is as per "Tips for Documentation Strings" in the elisp manual.
For consistency, do the same in diagnostics and comments.

lisp/emacs-lisp/bytecomp.el

@@ -348,7 +348,7 @@ else the global value will be modified."
 ;;;###autoload
 (defun byte-compile-enable-warning (warning)
   "Change `byte-compile-warnings' to enable WARNING.
-If `byte-compile-warnings' is `t', do nothing.  Otherwise, if the
+If `byte-compile-warnings' is t, do nothing.  Otherwise, if the
 first element is `not', remove WARNING, else add it.
 Normally you should let-bind `byte-compile-warnings' before calling this,
 else the global value will be modified."

lisp/emacs-lisp/check-declare.el

@@ -98,7 +98,7 @@ don't know how to recognize (e.g. some macros)."
                  (stringp (setq fnfile (nth 2 form)))
                  (setq fnfile (check-declare-locate fnfile
                                                     (expand-file-name file)))
-                 ;; Use `t' to distinguish unspecified arglist from empty one.
+                 ;; Use t to distinguish unspecified arglist from empty one.
                  (or (eq t (setq arglist (if (> len 3)
                                              (nth 3 form)
                                            t)))

lisp/emacs-lisp/checkdoc.el

@@ -1615,8 +1615,8 @@ function,command,variable,option or symbol." ms1))))))
 	    (or
 	     ;; * The documentation string for a variable that is a
 	     ;;   yes-or-no flag should start with words such as Non-nil
-	     ;;   means..., to make it clear that all non-`nil' values are
-	     ;;   equivalent and indicate explicitly what `nil' and non-`nil'
+	     ;;   means..., to make it clear that all non-nil values are
+	     ;;   equivalent and indicate explicitly what nil and non-nil
 	     ;;   mean.
 	     ;; * If a user option variable records a true-or-false
 	     ;;   condition, give it a name that ends in `-flag'.

lisp/emacs-lisp/cl-generic.el

@@ -525,7 +525,7 @@ which case this method will be invoked when the argument is `eql' to VAL.
           (progn
             (while (and dispatches
                         (let ((x (nth 1 (car dispatches))))
-                          ;; No need to dispatch for `t' specializers.
+                          ;; No need to dispatch for t specializers.
                           (or (null x) (equal x cl--generic-t-generalizer))))
               (setq dispatches (cdr dispatches)))
             (pop dispatches))))
@@ -701,7 +701,7 @@ methods.")
       #'cl--generic-standard-method-combination)
 
 (cl-defmethod cl-generic-generalizers (specializer)
-  "Support for the catch-all `t' specializer."
+  "Support for the catch-all t specializer."
   (if (eq specializer t) (list cl--generic-t-generalizer)
     (error "Unknown specializer %S" specializer)))
 

lisp/emacs-lisp/cl-indent.el

@@ -140,13 +140,13 @@ If non-nil, alignment is done with the first parameter
 
 (defcustom lisp-indent-backquote-substitution-mode t
   "How to indent substitutions in backquotes.
-If `t', the default, indent substituted forms normally.
-If `nil', do not apply special indentation rule to substituted
+If t, the default, indent substituted forms normally.
+If nil, do not apply special indentation rule to substituted
 forms.  If `corrected', subtract the `,' or `,@' from the form
 column, indenting as if this character sequence were not present.
 In any case, do not backtrack beyond a backquote substitution.
 
-Until Emacs 25.1, the `nil' behavior was hard-wired."
+Until Emacs 25.1, the nil behavior was hard-wired."
   :version "25.1"
   :type '(choice (const corrected) (const nil) (const t))
   :group 'lisp-indent)

lisp/emacs-lisp/debug.el

@@ -54,7 +54,7 @@ the middle is discarded, and just the beginning and end are displayed."
 The value affects the behavior of operations on any window
 previously showing the debugger buffer.
 
-`nil' means that if its window is not deleted when exiting the
+nil means that if its window is not deleted when exiting the
   debugger, invoking `switch-to-prev-buffer' will usually show
   the debugger buffer again.
 

lisp/emacs-lisp/edebug.el

@@ -600,7 +600,7 @@ list of a symbol.")
 (defun edebug-get-form-data-entry (pnt &optional end-point)
   ;; Find the edebug form data entry which is closest to PNT.
   ;; If END-POINT is supplied, match must be exact.
-  ;; Return `nil' if none found.
+  ;; Return nil if none found.
   (let ((rest edebug-form-data)
 	closest-entry
 	(closest-dist 999999))  ;; Need maxint here.

lisp/emacs-lisp/syntax.el

@@ -574,7 +574,7 @@ running the hook."
 
 ;; (defun buffer-syntactic-context (&optional buffer)
 ;;   "Syntactic context at point in BUFFER.
-;; Either of `string', `comment' or `nil'.
+;; Either of `string', `comment' or nil.
 ;; This is an XEmacs compatibility function."
 ;;   (with-current-buffer (or buffer (current-buffer))
 ;;     (syntax-ppss-context (syntax-ppss))))