magit-mode-map: bind S-tab to magit-section-cycle-global

It appears this was always the intention.  Keep the bindings
for s-tab and backtab, in case someone actually uses those.

Fixes #2759.

Documentation/magit.org

@@ -1017,7 +1017,7 @@ but all you need to get started are the next two.
 
   Cycle the visibility of diff-related sections in the current buffer.
 
-- Key: s-<tab>, magit-section-cycle-global
+- Key: S-<tab>, magit-section-cycle-global
 
   Cycle the visibility of all sections in the current buffer.
 

Documentation/magit.texi

@@ -1432,9 +1432,9 @@ Cycle the visibility of current section and its children.
 
 Cycle the visibility of diff-related sections in the current buffer.
 
-@kindex s-<tab>
+@kindex S-<tab>
 @cindex magit-section-cycle-global
-@item @kbd{s-<tab>} @tie{}@tie{}@tie{}@tie{}(@code{magit-section-cycle-global})
+@item @kbd{S-<tab>} @tie{}@tie{}@tie{}@tie{}(@code{magit-section-cycle-global})
 
 Cycle the visibility of all sections in the current buffer.
 @end table

lisp/magit-mode.el

@@ -288,6 +288,8 @@ starts complicating other things, then it will be removed."
     (define-key map "\t"    'magit-section-toggle)
     (define-key map [C-tab] 'magit-section-cycle)
     (define-key map [M-tab] 'magit-section-cycle-diffs)
+    (define-key map [S-tab] 'magit-section-cycle-global)
+    ;; Next two are for backward compatibility.
     (define-key map [s-tab] 'magit-section-cycle-global)
     (define-key map [backtab] 'magit-section-cycle-global)
     (define-key map "^"    'magit-section-up)