Update documentation with @TransactionalAdvice

[zsiegel]

Updating documentation to clarify an issue I ran into here #1285

This adds the @TransactionalAdvice annotation example to the section pertaining to usage of multiple datasources with repositories.

I also updated some links to the code annotations where they were not present.

[zsiegel]

Adding a rendering of the changes highlighted in red as the appear on my local output.

[dstepanov]

Thanks for the contribution!