Our database schema documentation previously didn't give any
indication of where this most essential piece of information is
stored.
* user in searching. But the database doesn't really care itself
* about any of these.
*
+ * Finally, the data portion of a mail document contains the path name
+ * of the mail message (relative to the database path).
+ *
* Directory document
* ------------------
* A directory document is used by a client of the notmuch library to