Prefer spelling "cacheable" over "cachable".

Previously we had both in code and comments.  Keep the more common and
accepted variant.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/5EBF1771-0566-4D08-9F9B-CDCDEF4BDC98@gmail.com

src/backend/access/heap/heapam.c

@@ -2248,7 +2248,7 @@ heap_insert(Relation relation, HeapTuple tup, CommandId cid,
 		ReleaseBuffer(vmbuffer);
 
 	/*
-	 * If tuple is cachable, mark it for invalidation from the caches in case
+	 * If tuple is cacheable, mark it for invalidation from the caches in case
 	 * we abort.  Note it is OK to do this after releasing the buffer, because
 	 * the heaptup data structure is all in local memory, not in the shared
 	 * buffer.
@@ -2700,7 +2700,7 @@ heap_multi_insert(Relation relation, TupleTableSlot **slots, int ntuples,
 	CheckForSerializableConflictIn(relation, NULL, InvalidBlockNumber);
 
 	/*
-	 * If tuples are cachable, mark them for invalidation from the caches in
+	 * If tuples are cacheable, mark them for invalidation from the caches in
 	 * case we abort.  Note it is OK to do this after releasing the buffer,
 	 * because the heaptuples data structure is all in local memory, not in
 	 * the shared buffer.

src/backend/utils/cache/catcache.c

@@ -829,7 +829,7 @@ ResetCatalogCachesExt(bool debug_discard)
  *	kinds of trouble if a cache flush occurs while loading cache entries.
  *	We now avoid the need to do it by copying cc_tupdesc out of the relcache,
  *	rather than relying on the relcache to keep a tupdesc for us.  Of course
- *	this assumes the tupdesc of a cachable system table will not change...)
+ *	this assumes the tupdesc of a cacheable system table will not change...)
  */
 void
 CatalogCacheFlushCatalog(Oid catId)