This probably should have been part of 3458e3c89c, but I missed it.
---
 lib/database.cc | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/lib/database.cc b/lib/database.cc
index 9630000..ecf576f 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -185,6 +185,14 @@ typedef struct {
  *			generated is 1 and the value will be
  *			incremented for each thread ID.
  *
+ *      C*              metadata keys starting with C indicate
+ *                      configuration data. It can be managed with the
+ *                      n_database_*config* API.  There is a convention
+ *			of hierarchical keys separated by '.' (e.g.
+ *			query.notmuch stores the value for the named
+ *			query 'notmuch'), but it is not enforced by the
+ *			API.
+ *
  * Obsolete metadata
  * -----------------
  *
-- 
2.8.1