Docs: improve description of bcbio_system
Docs: improve description of bcbio_system

Avoid overemphasizing editing of bcbio_system in favor of sample
specific resources. #2248

lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,vladsaveliev/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,lbeltrame/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen