[Parquet] Minor: Update comments in page decompressor (#8764)

# Which issue does this PR close?

- follow on to #8756

# Rationale for this change

@etseidl comments:
#8756 (comment)

> Not relevant to this PR, but I think this TODO has largely been
addressed by #8376 which enabled
skipping the decoding of the page statistics.

While I was in here, I also wanted to capture the learning based on
@mapleFU 's comment
#8756 (comment)

> The code looks good to me but the I don't know if the comment "not
compressed" can be replaced, if decompress_buffer is called and
decompressed_size == 0 , seems that it generally means something like
"this page only have levels, but not have non-null values"? ( Point me
out if I'm wrong)

# What changes are included in this PR?

Include some comments
# Are these changes tested?

No (there are no code changes)

# Are there any user-facing changes?

No, this is internal comments only. No code / behavior changes

Author: alamb

parquet/src/file/serialized_reader.rs

@@ -387,8 +387,6 @@ pub(crate) fn decode_page(
         can_decompress = header_v2.is_compressed.unwrap_or(true);
     }
 
-    // TODO: page header could be huge because of statistics. We should set a
-    // maximum page header size and abort if that is exceeded.
     let buffer = match decompressor {
         Some(decompressor) if can_decompress => {
             let uncompressed_page_size = usize::try_from(page_header.uncompressed_page_size)?;
@@ -398,6 +396,8 @@ pub(crate) fn decode_page(
             let decompressed_size = uncompressed_page_size - offset;
             let mut decompressed = Vec::with_capacity(uncompressed_page_size);
             decompressed.extend_from_slice(&buffer[..offset]);
+            // decompressed size of zero corresponds to a page with no non-null values
+            // see https://github.com/apache/parquet-format/blob/master/README.md#data-pages
             if decompressed_size > 0 {
                 let compressed = &buffer[offset..];
                 decompressor.decompress(compressed, &mut decompressed, Some(decompressed_size))?;