[doc] improve MI varobj introduction

[Vladimir Prus]

Daniel Jacobowitz wrote:

> On Tue, Dec 26, 2006 at 06:51:54PM +0300, Vladimir Prus wrote:
>> > I didn't really follow this thread, but I would recommend posting a
>> > revised version of the patch with any changes when you're ready to.
>> 
>> I don't have any planned changes so far, except for three-word editorial
>> change suggested by Nick.
> 
> Right, but a final patch is still helpful - it lets a reviewer
> (probably Eli) look at what you have without having to follow the
> discussion.

Here's the final patch.

- Volodya