diff --git a/riegeli/digests/digester_handle.h b/riegeli/digests/digester_handle.h
index 4bd19e2c..0441a82b 100644
--- a/riegeli/digests/digester_handle.h
+++ b/riegeli/digests/digester_handle.h
@@ -475,6 +475,12 @@ struct IsValidDigesterTarget<
 //   //
 //   // `OriginalDigestType` can be any type convertible to `DigestType` using
 //   // `DigestConverter`.
+//   //
+//   // Depending on the digester, `Digest()` can be more efficient if `Close()`
+//   // is called before.
+//   //
+//   // Many digesters support calling `Digest()` and then accepting more data
+//   // or calling `Digest()` again, but this is not guaranteed.
 //   OriginalDigestType Digest();
 // ```
 //
diff --git a/riegeli/digests/digesting_reader.h b/riegeli/digests/digesting_reader.h
index 4b728ae6..5afa03c0 100644
--- a/riegeli/digests/digesting_reader.h
+++ b/riegeli/digests/digesting_reader.h
@@ -146,6 +146,12 @@ class DigestingReader : public DigestingReaderBase {
   // Digests buffered data if needed, and returns the digest.
   //
   // The digest is converted to `DesiredDigestType` using `DigestConverter`.
+  //
+  // Depending on the digester, `Digest()` can be more efficient if `Close()` is
+  // called before.
+  //
+  // Many digesters support calling `Digest()` and then accepting more data or
+  // calling `Digest()` again, but this is not guaranteed.
   template <
       typename DesiredDigestType = DigestType,
       std::enable_if_t<HasDigestConverter<DigestType, DesiredDigestType>::value,
diff --git a/riegeli/digests/digesting_writer.h b/riegeli/digests/digesting_writer.h
index 928d5ef9..ed903f40 100644
--- a/riegeli/digests/digesting_writer.h
+++ b/riegeli/digests/digesting_writer.h
@@ -180,6 +180,12 @@ class DigestingWriter : public DigestingWriterBase {
   // Digests buffered data if needed, and returns the digest.
   //
   // The digest is converted to `DesiredDigestType` using `DigestConverter`.
+  //
+  // Depending on the digester, `Digest()` can be more efficient if `Close()` is
+  // called before.
+  //
+  // Many digesters support calling `Digest()` and then accepting more data or
+  // calling `Digest()` again, but this is not guaranteed.
   template <
       typename DesiredDigestType = DigestType,
       std::enable_if_t<HasDigestConverter<DigestType, DesiredDigestType>::value,