Apply suggestion and add endianness section

cpp/src/arrow/compute/row/row_encoder_internal.cc

@@ -159,7 +159,7 @@ Status FixedWidthKeyEncoder::Encode(const ExecValue& data, int64_t batch_length,
   };
   if (data.is_array()) {
     ArraySpan viewed = data.array;
-    // The original type might not FixedSizeBinaryType, but it would
+    // The original type might not be FixedSizeBinaryType, but it would
     // treat the input as binary data.
     auto view_ty = fixed_size_binary(byte_width_);
     viewed.type = view_ty.get();

cpp/src/arrow/compute/row/row_encoder_internal.h

@@ -143,6 +143,7 @@ struct ARROW_EXPORT VarLengthKeyEncoder : KeyEncoder {
   void AddLength(const ExecValue& data, int64_t batch_length, int32_t* lengths) override {
     if (data.is_array()) {
       int64_t i = 0;
+      ARROW_DCHECK_EQ(data.array.length, batch_length);
       VisitArraySpanInline<T>(
           data.array,
           [&](std::string_view bytes) {
@@ -288,10 +289,15 @@ struct ARROW_EXPORT NullKeyEncoder : KeyEncoder {
 ///    the column is null or not.
 /// 3. The "variable payload" encoding for the column, it would exists only
 ///    for non-null string/binary columns.
+///    For string/binary columns, the length of the payload is in
+///    "fixed width" part, and the binary contents are in the
+///    "variable payload" part.
 /// 4. Specially, if all columns in a row are null, the caller may decide
 ///    to refer to kRowIdForNulls instead of actually encoding/decoding
 ///    it using any KeyEncoder. See the comment for encoded_nulls_.
 ///
+/// The endianness of the encoded bytes is platform-dependent.
+///
 /// ## Null Type
 ///
 /// Null Type is a special case, it doesn't occupy any space in the