[C++][Parquet] Correct three comments on the DELTA_BINARY_PACKED scan

The coalescing guard is against values_remaining_current_mini_block_, so
what it needs room for is the rest of the current miniblock, not a whole
one; a run legitimately starts from a partly consumed miniblock.

"Four is the narrowest width measured to win" reused a word that means
bit width everywhere else in this file. The threshold is four lanes.

PrefixSumVectorAndTail claimed its deltas pin the stored width at every
step of its loop. At the top of the range the spread is all ones, which
as a signed delta is -1, so the frame absorbs it and the miniblock
stores width 1. Every stored width from 0 to the type's width is still
reached, so the coverage is unchanged; only the comment was wrong.

Comments only.

Author: sfc-gh-pgaur

cpp/src/parquet/decoder.cc

@@ -1471,9 +1471,9 @@ std::make_unsigned_t<T> PrefixSumDeltas(T* values, int num_values,
   // A vector scan only pays off once a register holds enough values to beat the
   // chain of additions it replaces. At two lanes it loses: one doubling step plus
   // carrying the running value across vectors costs more than the two additions it
-  // saves. Four is the narrowest width measured to win, so that is the threshold;
-  // where a register holds fewer, this loop is dropped and the one below does all
-  // the work.
+  // saves. Four lanes is the narrowest register measured to win, so that is the
+  // threshold; where a register holds fewer, this loop is dropped and the one below
+  // does all the work.
   if constexpr (kLanes >= 4) {
     // Broadcasts the last lane, which carries the running value into the next
     // vector without a round trip through a general-purpose register. Reading the
@@ -1684,7 +1684,7 @@ class DeltaBitPackDecoder : public TypedDecoderImpl<DType> {
   // width that has not been checked, including the non-conformant widths InitBlock
   // tolerates for extraneous miniblocks.
   uint32_t CoalescibleMiniBlocks(uint32_t values_available) const {
-    // Folding in a whole miniblock first requires room for the current one in full.
+    // Folding in a whole miniblock first requires room for the rest of the current one.
     if (values_available < values_remaining_current_mini_block_) {
       return 0;
     }

cpp/src/parquet/encoding_test.cc

@@ -2118,16 +2118,17 @@ TYPED_TEST(TestDeltaBitPackEncoding, MiniblockBitWidthRuns) {
 TYPED_TEST(TestDeltaBitPackEncoding, PrefixSumVectorAndTail) {
   // A decoder may accumulate the running total several deltas at a time, finishing
   // whatever does not fill a whole group one delta at a time and carrying the total
-  // from each group into the next. Walk every residual bit width, and at each one
+  // from each group into the next. Walk the residual bit widths, and at each one
   // enough lengths to leave every remainder such a group can leave, so each width is
   // decoded through the grouped path, through the remainder, and across the hand-off
   // between them.
   //
   // Deltas alternate between the frame and the widest value the width can hold above
-  // it. That pins the stored width, keeps a non-zero frame in play -- whose running
-  // multiple grows with the index, so a decoder that folds it in per group has to get
-  // that multiple right -- and wraps the running total repeatedly, which is where a
-  // grouped total and a value-at-a-time one part company if any term is signed.
+  // it, which drives the stored width across the whole range 0 to the type's width.
+  // It also keeps a non-zero frame in play -- whose running multiple grows with the
+  // index, so a decoder that folds it in per group has to get that multiple right --
+  // and wraps the running total repeatedly, where a grouped total and a
+  // value-at-a-time one disagree if any term is signed.
   using T = typename TypeParam::c_type;
   using UT = std::make_unsigned_t<T>;
   constexpr int kBits = static_cast<int>(sizeof(T) * 8);