Finish up XLOG_HINT renaming

Commit b8fd1a09f3 renamed XLOG_HINT to XLOG_FPI, but neglected two
places.

Backpatch to 9.3, like that commit.

src/backend/access/transam/README

@@ -624,7 +624,7 @@ of optimising for performance. When a hint is written we use
 MarkBufferDirtyHint() to mark the block dirty.
 
 If the buffer is clean and checksums are in use then
-MarkBufferDirtyHint() inserts an XLOG_HINT record to ensure that we
+MarkBufferDirtyHint() inserts an XLOG_FPI record to ensure that we
 take a full page image that includes the hint. We do this to avoid
 a partial page write, when we write the dirtied page. WAL is not
 written during recovery, so we simply skip dirtying blocks because

src/backend/storage/buffer/bufmgr.c

@@ -3015,7 +3015,7 @@ IncrBufferRefCount(Buffer buffer)
  * This is essentially the same as MarkBufferDirty, except:
  *
  * 1. The caller does not write WAL; so if checksums are enabled, we may need
- *	  to write an XLOG_HINT WAL record to protect against torn pages.
+ *	  to write an XLOG_FPI WAL record to protect against torn pages.
  * 2. The caller might have only share-lock instead of exclusive-lock on the
  *	  buffer's content lock.
  * 3. This function does not guarantee that the buffer is always marked dirty