Minor: Restore warning comment on Int96 statistics read (#7975)

# Which issue does this PR close?


- Follow on to #7687

# Rationale for this change

I merged #7687 without addressing
one of @emkornfield 's suggestions:
https://github.com/apache/arrow-rs/pull/7687/files#r2205393903

# What changes are included in this PR?

Implement the suggestion (restore a comment_

# Are these changes tested?

 BY CI

# Are there any user-facing changes?

No

Author: alamb

parquet/src/file/statistics.rs

@@ -209,6 +209,7 @@ pub fn from_thrift(
                     old_format,
                 ),
                 Type::INT96 => {
+                    // INT96 statistics may not be correct, because comparison is signed
                     let min = if let Some(data) = min {
                         assert_eq!(data.len(), 12);
                         Some(Int96::try_from_le_slice(&data)?)