This patch is another in my series of copy-edits to extend.texi.  This 
installment covers random problems with grammar, punctuation, 
terminology, etc that jumped out at me when reading through the chapter, 
rather than being a systematic search-and-destroy on pervasive usage 
problems (as most of the previous patches in this series were).  Since 
these changes are supposed to be content-free, I've checked this in.

I've probably reached the end of this sort of microscopic improvements 
to the text of this chapter.  However, I'm still kind of unhappy with 
the organization of the material and I may attempt to do some 
rearrangement to e.g. better group all the attribute discussion together.

-Sandra