[PATCH v2] jhbuildrc: Make configuration and usage easier

[Dirk Wallenstein]

Ah!!! stable documentation actually means obsolete documentation. I've
found the following mistakes (2.29 ist the real documentation [2]) :
- You can request branch switches in git.

I've taken two paragraphs (one in build_policy, and one in installprog)
almost verbatim from the jhbuild manual at [1]. I should probably note
that somewhere.

[1] http://library.gnome.org/devel/jhbuild/stable/
[2] http://library.gnome.org/devel/jhbuild/2.29/