Changes in dga.tex in response to comments from arp

Thanks to arp. See
http://stacks.math.columbia.edu/tag/065X#comment-283
http://stacks.math.columbia.edu/tag/09JI#comment-284
http://stacks.math.columbia.edu/tag/09JT#comment-285
http://stacks.math.columbia.edu/tag/09L9#comment-286
http://stacks.math.columbia.edu/tag/09LC#comment-287
http://stacks.math.columbia.edu/tag/09L9#comment-300