Cosmetic fixes for hash index write-ahead logging.

Amit Kapila.  One of these was reported by Tom Lane.

Discussion: http://postgr.es/m/5515.1489514099@sss.pgh.pa.us

src/backend/access/hash/README

@@ -365,9 +365,6 @@ try to split the bucket again until the prior split is finished.  In other
 words, a bucket can be in the middle of being split for some time, but it can't
 be in the middle of two splits at the same time.
 
-Although we can survive a failure to split a bucket, a crash is likely to
-corrupt the index, since hash indexes are not yet WAL-logged.
-
 The fourth operation is garbage collection (bulk deletion):
 
 	next bucket := 0

src/backend/access/hash/hashpage.c

@@ -975,7 +975,7 @@ fail:
  * hash indexes sequentially anyway, that probably doesn't matter.
  *
  * XXX It's annoying that this code is executed with the metapage lock held.
- * We need to interlock against _hash_getovflpage() adding a new overflow page
+ * We need to interlock against _hash_addovflpage() adding a new overflow page
  * concurrently, but it'd likely be better to use LockRelationForExtension
  * for the purpose.  OTOH, adding a splitpoint is a very infrequent operation,
  * so it may not be worth worrying about.