Fix various typos and incorrect/outdated name references

Author: Alexander Lakhin
Discussion: https://postgr.es/m/699beab4-a6ca-92c9-f152-f559caf6dc25@gmail.com

src/backend/utils/mmgr/generation.c

@@ -544,7 +544,7 @@ GenerationBlockInit(GenerationContext *context, GenerationBlock *block,
 
 /*
  * GenerationBlockIsEmpty
- *		Returns true iif 'block' contains no chunks
+ *		Returns true iff 'block' contains no chunks
  */
 static inline bool
 GenerationBlockIsEmpty(GenerationBlock *block)

src/backend/utils/sort/tuplesort.c

@@ -494,7 +494,7 @@ static void tuplesort_updatemax(Tuplesortstate *state);
  * abbreviations of text or multi-key sorts.  There could be!  Is it worth it?
  */
 
-/* Used if first key's comparator is ssup_datum_unsigned_compare */
+/* Used if first key's comparator is ssup_datum_unsigned_cmp */
 static pg_attribute_always_inline int
 qsort_tuple_unsigned_compare(SortTuple *a, SortTuple *b, Tuplesortstate *state)
 {
@@ -517,7 +517,7 @@ qsort_tuple_unsigned_compare(SortTuple *a, SortTuple *b, Tuplesortstate *state)
 }
 
 #if SIZEOF_DATUM >= 8
-/* Used if first key's comparator is ssup_datum_signed_compare */
+/* Used if first key's comparator is ssup_datum_signed_cmp */
 static pg_attribute_always_inline int
 qsort_tuple_signed_compare(SortTuple *a, SortTuple *b, Tuplesortstate *state)
 {
@@ -541,7 +541,7 @@ qsort_tuple_signed_compare(SortTuple *a, SortTuple *b, Tuplesortstate *state)
 }
 #endif
 
-/* Used if first key's comparator is ssup_datum_int32_compare */
+/* Used if first key's comparator is ssup_datum_int32_cmp */
 static pg_attribute_always_inline int
 qsort_tuple_int32_compare(SortTuple *a, SortTuple *b, Tuplesortstate *state)
 {

src/backend/utils/time/snapmgr.c

@@ -1630,7 +1630,7 @@ ThereAreNoPriorRegisteredSnapshots(void)
 }
 
 /*
- * HaveRegisteredOrActiveSnapshots
+ * HaveRegisteredOrActiveSnapshot
  *		Is there any registered or active snapshot?
  *
  * NB: Unless pushed or active, the cached catalog snapshot will not cause