[DOC] Add clear documentation for PMM MySQL user with and without SUPER privileges.

Currently, PMM1 and PMM2 documentation has the following details for SUPER privileges:

PMM2 Doc:  https://www.percona.com/doc/percona-monitoring-and-management/2.x/setting-up/client/mysql.html#setting-up-client-user

It is good practice to use a non-superuser account to connect PMM Client to the monitored database instance. This example creates a database user with name pmm, password pass, and the necessary permissions.

Expectation: Add Example without SUPER privileges and also add details what user can not do without SUPER privileges access.

Ref: https://forums.percona.com/t/super-privileges-for-pmm-user/6802/2

PMM1 Doc: https://www.percona.com/doc/percona-monitoring-and-management/1.x/conf-mysql.html#creating-a-mysql-user-account-to-be-used-with-pmm

When adding a MySQL instance to monitoring, you can specify the MySQL server superuser account credentials. However, monitoring with the superuser account is not secure. If you also specify the --create-user option, it will create a user with only the necessary privileges for collecting data.

You can also set up the pmm user manually with necessary privileges and pass its credentials when adding the instance.

The superuser credentials are required only to set up the pmm user with necessary privileges for collecting data.* If you want to create this user yourself, the following privileges are required: <--- This is bit confusing It says "If you want to create this user yourself" and in the example, it has SUPER privileges.

So if you already created a user for PMM in MySQL manually then SUPER privileges not needed.

Expectation: Correct the example