Updated misleading documentation about word_margin

[jstockwin]

Pull request

As per #390 (comment) and #390 (comment), updates the documentation for char_margin and word_margin.

Closes #390

I noticed that the docs in converting_pdf_to_text.rst were actually more correct in terms of what each parameter does (and this is now consistent in the other files). However I'm fairly sure some of the logic about making word_margin smaller/larger was slightly wrong, please check this as it does take some thinking about.. In my head, smaller word margin means there doesn't need to be as much of a gap between chars for a space to be inserted, i.e. you get more, shorter words.

Your comment also said to update tools/pdfdiff.py but actually the documentation here doesn't contain the detail. Perhaps it's taken from the other files or something?

How Has This Been Tested?

Documentation changes can't really be tested

Checklist

• I have added tests that prove my fix is effective or that my feature
  works (N/A)
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial
  version
• I have updated the README.md or I am verified that this
  is not necessary
• I have updated the readthedocs documentation or I
  verified that this is not necessary
• I have added a consice human-readable description of the change to
  CHANGELOG.md