doc-section.sh now uses ~/lilypond-git/

[Graham Percival]

I've changed scripts/auxiliar/doc-section.sh to use ~/lilypond-git/ by
default, since this was confusing some contributors and the change had
snuck into a patch they were proposing.

Sorry for the inconvenience if you use a different dir, but we should
make it simple for beginners and standardizing on the directory is a
good start.

Cheers,
- Graham