Doc: NR 4.1.2: Reorganize vertical dimensions. (issue2316042)

[markpolesky]

Reviewers: ,

Message:
The vertical dimensions docs in NR 4.1.2 "Page formatting"
are (in my opinion) incredibly confusing.  Here's a patch to
reorganize that section.  One big disclaimer: the
information here is presented according to *my*
understanding of how these variables were *intended* to
behave (or in some cases, how I think they *should* behave).

I'm fully aware that some of the text is verifiably
inaccurate, with respect to what the actual current behavior
is.  So, hold back the finger-pointing (for those so
inclined), and let's discuss this.  I'm not about to push
the patch impulsively.

- Mark

Description:
Doc: NR 4.1.2: Reorganize vertical dimensions.

Please review this at http://codereview.appspot.com/2316042/

Affected files:
  M Documentation/notation/spacing.itely