Remove int return from Update

While we're on the subject of cleaning the DocumentStore API, this int
return is also kind of awkwardly placed, and forces reads after every
write. It doesn't look like any of the actual application code cares
about the new version's number anyway.
Clarify Update MAX_CONTENT_SIZE

Update doesn't *have* to error out on large documents. A DocumentStore
may choose to support documents over the MAX_CONTENT_SIZE. However, the
API now notes that it must support those documents for both reads and
writes. For example, a DocumentStore is not allowed to just truncate
every document over a certain size. It should return an error, as noted,
if it can't support those large documents.
Fix capitalization

"Goose", not "goose". Consistency is important.