aio: Minor comment improvements

Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/usbwzckj7q3jhfx3ann3nrfnukmupbs35axvq5zfyeo6nvrzrm@onjhxs2du4st

src/backend/storage/smgr/md.c

@@ -1981,7 +1981,7 @@ md_readv_complete(PgAioHandle *ioh, PgAioResult prior_result, uint8 cb_data)
 		 * might not process the query result immediately (because it is busy
 		 * doing another part of query processing) or at all (e.g. if it was
 		 * cancelled or errored out due to another IO also failing).  The
-		 * issuer of the IO will emit an ERROR when processing the IO's
+		 * definer of the IO will emit an ERROR when processing the IO's
 		 * results
 		 */
 		pgaio_result_report(result, td, LOG_SERVER_ONLY);

src/backend/storage/smgr/smgr.c

@@ -738,6 +738,14 @@ smgrreadv(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
  * blocks not successfully read might bear unspecified modifications, up to
  * the full nblocks). This maintains the abstraction that smgr operates on the
  * level of blocks, rather than bytes.
+ *
+ * Compared to smgrreadv(), more responsibilities fall on the caller:
+ * - Partial reads need to be handled by the caller re-issuing IO for the
+ *   unread blocks
+ * - smgr will ereport(LOG_SERVER_ONLY) some problems, but higher layers are
+ *   responsible for pgaio_result_report() to mirror that news to the user (if
+ *   the IO results in PGAIO_RS_WARNING) or abort the (sub)transaction (if
+ *   PGAIO_RS_ERROR).
  */
 void
 smgrstartreadv(PgAioHandle *ioh,