Fix typos and grammar in the code

The large majority of these have been introduced by recent commits done
in the v18 development cycle.

Author: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/9a7763ab-5252-429d-a943-b28941e0e28b@gmail.com

src/backend/storage/aio/README.md

@@ -103,7 +103,7 @@ pgaio_io_set_handle_data_32(ioh, (uint32 *) buffer, 1);
  *
  * E.g. md.c needs to translate block numbers into offsets in segments.
  *
- * Once the IO handle has been handed off to smgstartreadv(), it may not
+ * Once the IO handle has been handed off to smgrstartreadv(), it may not
  * further be used, as the IO may immediately get executed below
  * smgrstartreadv() and the handle reused for another IO.
  *

src/backend/storage/aio/method_worker.c

@@ -321,7 +321,7 @@ pgaio_worker_die(int code, Datum arg)
 }
 
 /*
- * Register the worker in shared memory, assign MyWorkerId and register a
+ * Register the worker in shared memory, assign MyIoWorkerId and register a
  * shutdown callback to release registration.
  */
 static void

src/backend/storage/buffer/bufmgr.c

@@ -4970,7 +4970,7 @@ FlushRelationBuffers(Relation rel)
 				ResourceOwnerEnlarge(CurrentResourceOwner);
 
 				/*
-				 * Pin/upin mostly to make valgrind work, but it also seems
+				 * Pin/unpin mostly to make valgrind work, but it also seems
 				 * like the right thing to do.
 				 */
 				PinLocalBuffer(bufHdr, false);

src/backend/storage/page/bufpage.c

@@ -88,7 +88,7 @@ PageInit(Page page, Size pageSize, Size specialSize)
  * To allow the caller to report statistics about checksum failures,
  * *checksum_failure_p can be passed in. Note that there may be checksum
  * failures even if this function returns true, due to
- * IGNORE_CHECKSUM_FAILURE.
+ * PIV_IGNORE_CHECKSUM_FAILURE.
  */
 bool
 PageIsVerified(PageData *page, BlockNumber blkno, int flags, bool *checksum_failure_p)