Consider using the COMMENT= clause to document why a particular change was made the next time you make a change to a parameter using an SPFILE:

SQL> alter system set pga_aggregate_target=512m comment='Changed 04-JUN-2018, AWR recommendation, MR';

System altered.

 

SQL> select value, update_comment from v$parameter where name = 'pga_aggregate_target';SQL> select value, update_comment from v$parameter where name = 'pga_aggregate_target';
VALUE      UPDATE_COMMENT
-------------------- ---------------------------------------------------------------------------------------------------
536870912  Changed 04-JUN-2018, AWR recommendation, MR