Update ALTER AVAILABILITY GROUP documentation as sysadmin

Clarified the requirement to run the command as 'sysadmin' on both primary and secondary availability group instances.

Author: JamesFerebee

docs/t-sql/statements/alter-availability-group-transact-sql.md

@@ -676,7 +676,7 @@ Specifies manual seeding. This method requires you to create a backup of the dat
 Modifies any of the availability group settings of a distributed availability group. The list of availability groups to modify contains the availability group name and a `WITH (...)` clause for each availability group.
 
 > [!IMPORTANT]  
-> You must run this command on both the primary availability group and secondary availability group instances.
+> You must run this command on both the primary availability group and secondary availability group instances as `sysadmin`. 
 
 #### GRANT CREATE ANY DATABASE