Fix display of menu-bar bindings of commands in *Help* buffers

Fix display of menu-bar bindings of commands in *Help* buffers

* lisp/help-fns.el (help-fns--insert-menu-bindings): Propertize
with 'help-key-binding' face only the menu items, not the arrows
between successive items.  This is because 'char-displayable-p' is
unreliable when we propertize the character with an arbitrary
face: that face could specify a font which doesn't support the
character after all, while 'char-displayable-p' assumes there are
no restrictions on fonts that can be used for displaying the
character.  Also, make the code more efficient by avoiding the
call to 'char-displayable-p' inside the loop.