Update collect[] param documentation (#300)
Update collect[] param documentation (#300)

Update the `collect[]` param documentation to match the `node_exporter`
docs. This avoids new user copy-pasta issues.

Signed-off-by: Ben Kochie <9bbf917dc63fbaa40c67921332d26cf16a50db75@gmail.com>
roman-vynar/mysqld_exporter,prometheus/mysqld_exporter,prometheus/mysqld_exporter,roman-vynar/mysqld_exporter