Add that changing the `argname` will raise exception in procedure/function

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/12/sql-createprocedure.html
Description:

In https://www.postgresql.org/docs/12/sql-createprocedure.html you can add:
"It is also not possible to change the argname. Trying to do so will raise
an exception. If you want to do it - drop the procedure first."

In https://www.postgresql.org/docs/12/sql-createfunction.html you can add:
"It is also not possible to change the argname. Trying to do so will raise
an exception. If you want to do it - drop the function first."