Describe why a multiple foreign keys should not be created with a new table

What does this MR do?

Add a couple of sentences to the Migration Style Guide describing why multiple foreign keys should not be created with a new table.

Why is this change being made?

This threw me off in a recent database review because the docs don't go into detail about why we recommend creating foreign keys separately. For the benefit of others, we can say why, not just what.

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• 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?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• [-] This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• [-] This change is backwards compatible across updates, or this does not apply.