docs(store/protos): update comment on blobstore Read and Put

Further emphasize Read() can be used to ask for blobs OR chunks, and
that clients usually want to stat and then request (smaller) chunks,
rather than reading whole blobs.

Also clarify that the chunking used to send BlobChunks over has nothing
to do with the chunk sizes communicated in a Stat() request.

Change-Id: Ia615d190aae570611de2655b11342a14d0b75976
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8028
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/store/protos/rpc_blobstore.pb.go

@@ -161,7 +161,7 @@ type ReadBlobRequest struct {
 	sizeCache     protoimpl.SizeCache
 	unknownFields protoimpl.UnknownFields
 
-	// The blake3 digest of the blob requested
+	// The blake3 digest of the blob or chunk requested
 	Digest []byte `protobuf:"bytes,1,opt,name=digest,proto3" json:"digest,omitempty"`
 }