clean up text

[edwardhartnett]

The explanatory text is very wordy. Some suggestions:

• Remove unneeded phrases like "Nevertheless", "Therefore", etc. They are appropriate for spoken English but not technical documentation.
• Long paragraphs need to be broken up.
• Long sentences need to be broken up.
• Better use of bullet and numbered lists would help.

In this PR I take an initial stab are removing unneeded text and adding paragraph and sentence breaks. Much more can be done but I think these changes make the documentation more readable to a newbie like myself.

Part of #397

[jbathegit]

I don't necessarily agree with all of the proposed changes, but then again I'm also not a newbie, so I'll defer to your judgement if you feel that this helps make everything clearer to a reader such as yourself ;-)