-/* Open the named notmuch configuration file. A filename of NULL will
- * be interpreted as the default configuration file
- * ($HOME/.notmuch-config).
+/* Open the named notmuch configuration file. If the filename is NULL,
+ * the value of the environment variable $NOTMUCH_CONFIG will be used.
+ * If $NOTMUCH_CONFIG is unset, the default configuration file
+ * ($HOME/.notmuch-config) will be used.