-/* Return the SHA-1 message digest of the data of the specified stream, or a
- * void SHA-1 of all zeroes if the specified stream is empty. */
+/*
+ * Return the SHA-1 message digest of the data of the specified stream, or a
+ * void SHA-1 of all zeroes if the specified stream is empty, or NULL if the
+ * specified stream is unhashed. (Most callers ensure the stream cannot be
+ * unhashed.)
+ */