sql: add FLUSH PRIVILEGES hint (#835)

* Add FLUSH PRIVILEGES hint to user-account-management.md and FAQ.md

See photoprism/photoprism#60 (comment)

* FLUSH PRIVILEGES only required if grant table was modified

* Fix wording: grant tables

* Change wording as requested

* Change link as requested

* Use backticks to format title as code

Co-Authored-By: lastzero <michael@lastzero.net>

sql/user-account-management.md

@@ -93,3 +93,13 @@ Or:
 ```sql
 ALTER USER 'jeffrey'@'localhost' IDENTIFIED BY 'mypass';
 ```
+
+## `FLUSH PRIVILEGES` 
+
+If you modified the grant tables directly, run the following command to apply changes immediately:
+
+```sql
+FLUSH PRIVILEGES;
+```
+
+For details, see [Privilege Management](../sql/privilege.md).