PG-1013 Updated the functions document (#370)

* PG-1013 Updated the functions document

* Removed the pg_alter_keyring_provider

---------

Co-authored-by: Artem Gavrilov <artem.gavrilov@percona.com>

documentation/docs/functions.md

@@ -72,12 +72,19 @@ SELECT pg_tde_rotate_principal_key('name-of-the-new-principal-key', NULL);
 SELECT pg_tde_rotate_principal_key(NULL, 'name-of-the-new-provider');
 ```
 
+
 ## pg_tde_is_encrypted
 
-Tells if a table is using the `pg_tde` access method or not.
+Tells if a table is encrypted using the `tde_heap` access method or not.
+
+To verify a table encryption, run the following statement:
 
 ```
 SELECT pg_tde_is_encrypted('table_name');
 ```
 
+You can also verify if the table in a custom schema is encrypted. Pass teh schema name for the function as follows:
 
+```
+SELECT pg_tde_is_encrypted('schema.table_name');
+```