Improve description of checks on backend

Actual result:

We are returning the following description for checks:

• MongoDB Authentication - description: This check returns a warning if MongoDB authentication is disabled.

• MongoDB Version - description: This check returns warnings if MongoDB/PSMDB version is not the latest one.

• MYSQL Anonymous Users - description: This check returns a warning if there are anonymous users.

• MYSQL Empty Password - description: This check returns an error if there are users without passwords.

• MYSQL Version - description: This check returns warnings if MySQL/PS/MariaDB version is not the latest one.

• PostgeSQL Super Role - description: This check returns a notice if there are users with superuser.

• PostgreSQL Version - description: This check returns warnings if PostgreSQL minor version is not the latest one.

As we are going to show the description on UI so there are several suggestions for improvement:

1. Not to use acronyms for databases e.g. PS - Percona Server for MySQL, PSMDB - Percona Server for MongoDB;
2. Fix typo in "PostgeSQL Super Role";
3. Update the description for PostgreSQL Super Role - "This check returns a notice if there are users with superuser role."