Add explanation for renaming sources to hover text

This is a (partial?) fix for #558. It adds an explanation of why, as a journalist, you might want to change a particular source's codename. Since I recently re-worked the UI for re-generating codenames (in the manner of the "additional improvements" mentioned in #558), I've only added this additional explanation to the hover text of the "Change codename" link.

We should also, at a minimum, mention the reasoning for changing the codename in the documentation for journalists (`docs/user_manual.md`).