update data service docs on multiple datasource

[musketyr]

The guide about multi-data source is properly explaining how to use custom data source with data services. None of this information is mentioned in the official documentation.

[niravassar]

This is ready for a review with a PR

[musketyr]

thanks, @niravassar. the PR looks good to me!

[niravassar]

i closed and deleted PR grails/grails-data-hibernate5#212

The PR to use is grails/grails-data-hibernate5#213

[erichelgeson]

Merged, thanks!