Make it clear that the fixity block may not contain fixity for all files in an object

[zimeon]

Split from #406: we need to make it clear that the fixity block may not contain fixity for all files in a version or object. The manifest serves this purpose, MUST be complete, and is the canonical fixity information for all content files. The fixity block is an optional convenience for storing additional fixity values, typically from migrations. It is also optional whether fixity information is carried forward from one version to another.