Distances and measurements

[Patrick McCarty]

Hi Trevor,

I noticed your doc update for "Distances and measurements", and it
looks really great!

I have two suggestions to further improve this section:

1) The `pt' unit is actually 1/72.27 inch, which is the TeX point.

2) Staff-space can be used as a unit too in the \paper and \layout blocks:

\paper {
  top-margin = 5\staff-space
  bottom-margin = 10\staff-space
}

Thanks,
Patrick