Add some hints for omitted proofs of basic properties

Thanks to Kestutis Cesnavicius
http://stacks.math.columbia.edu/tag/01K6#comment-646
http://stacks.math.columbia.edu/tag/03HF#comment-647
http://stacks.math.columbia.edu/tag/03HG#comment-648
http://stacks.math.columbia.edu/tag/03WH#comment-649
http://stacks.math.columbia.edu/tag/03WI#comment-650