Document allow_cross_joins_across_databases

What does this MR do?

This MR updates the Removing joins between ci_* and non ci_* tables section of the Multiple Databases developer documentation to include details on how to use the to ::Gitlab::Database.allow_cross_joins_across_databases() method to Explicitly allow the cross join across databases.

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Conformity

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• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
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• This change is backwards compatible across updates, or this does not apply.