Explain how one can override the connection options in the docs

[DavideD]

Sometime Hibernate Reactive is not able to parse the URL for the connection and sometimes people need advance configuration for the Vert.x SQL client.

We should add an example to the documentation about overriding the DefaultSqlClientPool, similar to what we explain for VertxInstance (probably we should mention that it's possible to override services).

See this answer on StackOverflow for reference.

[DavideD]

Kinda related to this other issue: #1425

[DavideD]

We already mention this in the docs: https://hibernate.org/reactive/documentation/2.0/reference/html_single/#_custom_connection_management_and_multitenancy

In the end, I think it's good enough