Fix typos, update README.

Peter Geoghegan

src/backend/lib/README

@@ -3,6 +3,8 @@ in the backend:
 
 binaryheap.c - a binary heap
 
+hyperloglog.c - a streaming cardinality estimator
+
 pairingheap.c - a pairing heap
 
 rbtree.c - a red-black tree

src/backend/utils/adt/varlena.c

@@ -2088,7 +2088,7 @@ bttext_abbrev_convert(Datum original, SortSupport ssup)
 	 *
 	 * First, Hash key proper, or a significant fraction of it.  Mix in length
 	 * in order to compensate for cases where differences are past
-	 * CACHE_LINE_SIZE bytes, so as to limit the overhead of hashing.
+	 * PG_CACHE_LINE_SIZE bytes, so as to limit the overhead of hashing.
 	 */
 	hash = hash_any((unsigned char *) authoritative_data,
 					Min(len, PG_CACHE_LINE_SIZE));

src/include/utils/sortsupport.h

@@ -165,8 +165,8 @@ typedef struct SortSupportData
 	 * may set it to NULL to indicate abbreviation should not be used (which is
 	 * something sortsupport routines need not concern themselves with).
 	 * However, sortsupport routines must not set it when it is immediately
-	 * established that abbreviation should not proceed (for abbreviation
-	 * calls, or platform-specific impediments to using abbreviation).
+	 * established that abbreviation should not proceed (e.g., for !abbreviate
+	 * calls, or due to platform-specific impediments to using abbreviation).
 	 */
 	Datum			(*abbrev_converter) (Datum original, SortSupport ssup);