docs: GRegex: @start_position is in bytes
docs: GRegex: @start_position is in bytes

The start_position arguments are passed to pcre_exec() as the
startoffset, which is in bytes (not characters).

I had recently a doubt about this, so it's better to document it.

https://bugzilla.gnome.org/show_bug.cgi?id=747927

mzabaluev/glib,MathieuDuponchelle/glib,Distrotech/glib,endlessm/glib,endlessm/glib,mzabaluev/glib,johne53/MB3Glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,johne53/MB3Glib,MathieuDuponchelle/glib,Distrotech/glib,mzabaluev/glib,johne53/MB3Glib,endlessm/glib,MathieuDuponchelle/glib,johne53/MB3Glib,Distrotech/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,mzabaluev/glib,mzabaluev/glib,endlessm/glib,Distrotech/glib,Distrotech/glib