lib: Fix slight misinformation in the database schema doc

The database schema documentation made it sound like each mail
document had exactly one on-disk message file, which hasn't been true
for a long time.

1 files changed, 6 insertions, 3 deletions

diff --git a/lib/database.cc b/lib/database.cc
index 45c42607..c7602906 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -54,9 +54,12 @@ typedef struct {
  *
  * Mail document
  * -------------
- * A mail document is associated with a particular email message file
- * on disk. It is indexed with the following prefixed terms which the
- * database uses to construct threads, etc.:
+ * A mail document is associated with a particular email message. It
+ * is stored in one or more files on disk (though only one has its
+ * content indexed) and is uniquely identified  by its "id" field
+ * (which is generally the message ID). It is indexed with the
+ * following prefixed terms which the database uses to construct
+ * threads, etc.:
  *
  *    Single terms of given prefix:
  *