Client-spec: How many tlog entries should clients expect?

[woodruffw]

From sigstore/sigstore-ruby#94 (comment) -- sigstore-python (and soon sigstore-ruby) both expect exactly one tlog entry per bundle, since the semantics for handling multiple entries are unclear (i.e. whether it's a K-of-N threshold or "all or none"). The client spec should probably clarify which of these (or both, contextually) of these is expected for Sigstore clients.