HashingWriter: Add documentation
HashingWriter: Add documentation

restic/restic,byo/restic,stuertz/restic,mappu/restic,stuertz/restic,ar-jan/restic,middelink/restic,jayme-github/restic,ar-jan/restic,ckemper67/restic,fawick/restic,howeyc/restic,jayme-github/restic,bchapuis/restic,byo/restic,fawick/restic,arithmetric/restic,middelink/restic,kurin/restic,kurin/restic,restic/restic,episource/restic