Clarify key deletion funcs description in docs

Key deletion fucntions don't delete anything as keys stored in external
key management system. So these functions just remove keys from TDE.

Author: artemgavrilov

contrib/pg_tde/documentation/docs/architecture/index.md

@@ -305,10 +305,10 @@ You can manage a default key with the following functions:
 
 #### Delete a key
 
-The `pg_tde_delete_key()` function removes the principal key for the current database. If the current database has any encrypted tables, and there isn’t a default principal key configured, it reports an error instead. If there are encrypted tables, but there’s also a default principal key, internal keys will be encrypted with the default key.
+The `pg_tde_delete_key()` function unsets the principal key for the current database. If the current database has any encrypted tables, and there isn’t a default principal key configured, it reports an error instead. If there are encrypted tables, but there’s also a default principal key, internal keys will be encrypted with the default key.
 
 !!! note
-    WAL keys **cannot** be deleted, as server keys are managed separately.
+    WAL keys **cannot** be unset, as server keys are managed separately.
 
 ### Current key details
 

contrib/pg_tde/documentation/docs/functions.md

@@ -335,15 +335,15 @@ The `ensure_new_key` parameter instructs the function how to handle a principal
 
 ### pg_tde_delete_key
 
-Deletes the principal key for the current database. If the current database has any encrypted tables, and there isn’t a default principal key configured, it reports an error instead. If there are encrypted tables, but there’s also a default principal key, internal keys will be encrypted with the default key.
+Unsets the principal key for the current database. If the current database has any encrypted tables, and there isn’t a default principal key configured, it reports an error instead. If there are encrypted tables, but there’s also a default principal key, internal keys will be encrypted with the default key.
 
 ```sql
 SELECT pg_tde_delete_key();
 ```
 
 ### pg_tde_delete_default_key
 
-Deletes default principal key. It's possible only if no database uses default principal key.
+Unsets default principal key. It's possible only if no database uses default principal key.
 
 ```sql
 SELECT pg_tde_delete_default_key();