mysql-*: improve READMEs

[NavidSassan]

The following section in the READMEs is unclear:

Requires a user account with high privileges to access schemas like INFORMATION_SCHEMA. For most INFORMATION_SCHEMA tables, each MySQL user has the right to access them, but can see only the rows in the tables that correspond to objects for which the user has the proper access privileges. <https://dev.mysql.com/doc/refman/5.7/en/information-schema-introduction.html#information-schema-privileges>_. So you can't grant permission to INFORMATION_SCHEMA directly, you have to grant permission to the tables on your own schemas, and as you do, those tables will start showing up in INFORMATION_SCHEMA queries <https://stackoverflow.com/questions/60499772/cannot-grant-mysql-user-access-to-information-schema-database>_. Then this check provide correct results.

The checks return data with only *.*:SELECT,PROCESS privileges, but it is unclear if those are enough ("high privileges?"), and if the data returned is correct or not (the last sentence implies that the check might return incorrect data with the wrong privileges). It also conflicts with "Requirements", "User with no privileges, ....

Found in:

• mysql-database-metrics
• mysql-innodb-buffer-pool-size
• mysql-storage-engines
• mysql-table-definition-cache
• mysql-table-indexes

[markuslf]

Tested and updated:

• [x] mysql-aria
• [x] mysql-database-metrics
• [x] mysql-innodb-buffer-pool-size
• [x] mysql-storage-engines
• [x] mysql-table-definition-cache
• [x] mysql-table-indexes