odb: document git_odb_stream

Clarify the role of each function and in particular mention that there
is no need for the backend or stream to worry about the object's id,
as it will be given when `finalize_write` is called.

include/git2/sys/odb_backend.h

@@ -48,6 +48,10 @@ struct git_odb_backend {
 	int (* read_header)(
 		size_t *, git_otype *, git_odb_backend *, const git_oid *);
 
+	/**
+	 * Write an object into the backend. The id of the object has
+	 * already been calculated and is passed in.
+	 */
 	int (* write)(
 		git_odb_backend *, const git_oid *, const void *, size_t, git_otype);