Document database setting

... and add null as possible values.
Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>

zathura.c

@@ -208,12 +208,12 @@ zathura_init(zathura_t* zathura)
     zathura->database = zathura_sqldatabase_new(tmp);
     g_free(tmp);
 #endif
-  } else {
+  } else if (g_strcmp0(database, "null") != 0) {
     girara_error("Database backend '%s' is not supported.", database);
   }
   g_free(database);
 
-  if (zathura->database == NULL) {
+  if (zathura->database == NULL && g_strcmp0(database, "null") != 0) {
     girara_error("Unable to initialize database. Bookmarks won't be available.");
   } else {
     g_object_set(zathura->ui.session->command_history, "io", zathura->database, NULL);

zathurarc.5.rst

@@ -634,6 +634,15 @@ Defines if the number of pages per row should be honored when advancing a page.
 * Value type: Boolean
 * Default value: false
 
+database
+^^^^^^^^
+Defines the database backend to use for bookmarks and input history. Possible
+values are "plain", "sqlite" (if built with sqlite support) and "null". If
+"null" is used, bookmarks and input history will not be stored.
+
+* Value type: String
+* Default value: plain
+
 incremental-search
 ^^^^^^^^^^^^^^^^^^
 En/Disables incremental search (search while typing).