[doc] slightly improve documentation of SQL engines

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/engines/sqlite.py

@@ -41,8 +41,13 @@ import sqlite3
 import contextlib
 
 engine_type = 'offline'
+
 database = ""
+"""Filename of the SQLite DB."""
+
 query_str = ""
+"""SQL query that returns the result items."""
+
 limit = 10
 paging = True
 result_template = 'key-value.html'