kong: update migrations instructions for Kong 1.0+

kong/content.md

@@ -46,12 +46,12 @@ docker run --rm \
     -e "KONG_DATABASE=postgres" \
     -e "KONG_PG_HOST=kong-database" \
     -e "KONG_CASSANDRA_CONTACT_POINTS=kong-database" \
-    %%IMAGE%% kong migrations up
+    %%IMAGE%% kong migrations bootstrap
 ```
 
 In the above example, both Cassandra and PostgreSQL are configured, but you should update the `KONG_DATABASE` environment variable with either `cassandra` or `postgres`.
 
-**Note**: migrations should never be run concurrently; only one Kong node should be performing migrations at a time.
+**Note for Kong < 0.15**: with Kong versions below 0.15 (up to 0.14), use the `up` sub-command instead of `bootstrap`. Also note that with Kong < 0.15, migrations should never be run concurrently; only one Kong node should be performing migrations at a time. This limitation is lifted for Kong 0.15, 1.0, and above.
 
 ### Start Kong