Change "XREF" to "Xref" in mode lighter

* lisp/progmodes/xref.el (xref--xref-buffer-mode)
(xref--transient-buffer-mode, xref-edit-save-changes):
Change "XREF" to "Xref" for internal consistency.

* doc/emacs/maintaining.texi (Xref Commands):
Same.

doc/emacs/maintaining.texi

@@ -2764,10 +2764,10 @@ Table}.)
 @node Xref Commands
 @subsubsection Commands Available in the @file{*xref*} Buffer
 @cindex commands in @file{*xref*} buffers
-@cindex XREF mode
+@cindex Xref mode
 
   The following commands are provided in the @file{*xref*} buffer by
-the special XREF mode:
+the special Xref mode:
 
 @table @kbd
 @item @key{RET}
@@ -2880,7 +2880,7 @@ prompt always, customize the value of the variable
 to prompt only if there's no usable identifier at point.)  The command
 then presents the @file{*xref*} buffer with all the references to the
 identifier, showing the file name and the line where the identifier is
-referenced.  The XREF mode commands are available in this buffer, see
+referenced.  The Xref mode commands are available in this buffer, see
 @ref{Xref Commands}.
 
 When invoked in a buffer whose major mode uses the @code{etags} backend,

lisp/progmodes/xref.el

@@ -629,7 +629,7 @@ If SELECT is non-nil, select the target window."
     (run-hooks 'xref-after-jump-hook)))
 
 
-;;; XREF buffer (part of the UI)
+;;; Xref buffer (part of the UI)
 
 ;; The xref buffer is used to display a set of xrefs.
 (defconst xref-buffer-name "*xref*"
@@ -1011,7 +1011,7 @@ point."
 (declare-function outline-search-text-property "outline"
                   (property &optional value bound move backward looking-at))
 
-(define-derived-mode xref--xref-buffer-mode special-mode "XREF"
+(define-derived-mode xref--xref-buffer-mode special-mode "Xref"
   "Mode for displaying cross-references."
   (setq buffer-read-only t)
   (setq next-error-function #'xref--next-error-function)
@@ -1041,7 +1041,7 @@ point."
 
 (define-derived-mode xref--transient-buffer-mode
   xref--xref-buffer-mode
-  "XREF Transient.")
+  "Xref Transient")
 
 (defun xref--imenu-prev-index-position ()
   "Move point to previous line in `xref' buffer.
@@ -1548,7 +1548,7 @@ The only editable texts in an Xref-Edit buffer are the match results."
   (use-local-map xref--xref-buffer-mode-map)
   (setq buffer-read-only t)
   (setq major-mode 'xref--xref-buffer-mode)
-  (setq mode-name "XREF")
+  (setq mode-name "Xref")
   (force-mode-line-update)
   (buffer-disable-undo)
   (setq buffer-undo-list t)