[doc, committed] more attribute documentation cleanup

[Sandra Loosemore]

I've checked in another installment of my ongoing cleanup of the 
attribute documentation in extend.texi.  In this patch, I've tidied up 
the variable and type attribute sections.  There's not much actual 
change to the document text here, mostly just moving things around and 
tweaking structure.  These two sections were in much better shape 
(besides being much shorter) than the function attribute section was 
that I tackled in my last patch.

Next I am planning to reorder the attribute sections within the chapter 
to group them together, and re-work the introductory material so that 
the presentation flows better.

-Sandra

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