Docs: improve description of capture BED files
Docs: improve description of capture BED files

Better organize and summarize usage of different capture BED files
for analysis and coverage QC. #1652

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