Improve the database configuration comments in settings.php.

[bugfolder]

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It's been suggested that we add some description of the array format for database configuration in settings.php. See discussion starting here (and links therein):

https://github.com/backdrop/backdrop-issues/issues/2231#issuecomment-1904857316

Currently in the docblock for $database, there is a link to "advanced database documentation" at https://api.backdropcms.org/database-configuration. However, this currently redirects to https://docs.backdropcms.org/documentation/database-configuration, and so at the very least, we should update the link.

As for whether to add some commentary about the array format directly (rather than relying on click-through to the advanced docs), reasonable minds may differ, but I think the penalty of the extra reading material would be worth increasing people's awareness of the existence of of the array format. So perhaps giving a single example of the array format in the docblock, then directing the reader to the documentation page for the details.