give --index-search example on man page

[積丹尼 Dan Jacobson]

On the info man page, after

       info emacs buffers
              start at buffers node within emacs manual

Please add

       info grub commands 'command-line and menu entry commands' badram
       info --index-search=badram grub
              start at badram node within grub manual

So one can realize it is not so simple.

Also consider making an -i shortcut to reduce the pain.

(I assume this is the equivalent of
(info "(grub) badram")
in emacs.)