Improve statement and clarify proof of lemma in coherent.tex

Thanks to correction_bot
http://stacks.math.columbia.edu/tag/01XL#comment-938
Fix typos in coherent.tex

Thanks to correction_bot
http://stacks.math.columbia.edu/tag/01XX#comment-940
Bunch of small mistakes fixed

Thanks to correction_bot
Comments 943 -- 953 on website
Finite nr of primes implies small dimension

Added some missing lemmas on this in order to have something
to refer to in a later result.

Thanks to JuanPablo.
http://stacks.math.columbia.edu/tag/02ML#comment-954