message: Added minor comment on DocId
diff: @@ -234,6 +234,8 @@ pub mod merge_policy {
  /// A `u32` identifying a document within a segment.
  /// Documents have their `DocId` assigned incrementally,
  /// as they are added in the segment.
  +///
  +/// At most, a segment can contain 2^31 documents.
  pub type DocId = u32;
  /// A u64 assigned to every operation incrementally
  