Update argument per comment by Bill.

  10 MB was an arbitrarly large number, 50Kb is better supported threshold.
Correct para in 2.1.1 per Bill's comments

The HIERARCH convention is actually following a loophole in
the standard, so it doesnt really support the argument in para
5
Remove NOAO, STSci error model text. Unreferenced

so its not supported. Argument doesnt really need it anyways.
Remove redded out text at the end of lession 3

(section 3.3). While I like the idea, it does not flow
with the argument contained in the section. Not enough
substance in the argument to make it a separate section
either.
Try to clarify the wording in para 3, lesson 3