Clarify BYO SQL instructions

[therealmitchconnors]

The FAQ indicates that you can bring your own instance of MySQL to use with the Operator, instead of relying on the (broken for me) mysql operator.

I can also see that docs/examples/full.yaml seems to imply that an 'externalSecret' value can be set to indicate the password of the external database, but there is nowhere to put the address, and that value does not appear to be referenced anywhere else in the codebase...