Improve clarity about SolidStart's support for database sessions

[amirhhashemi]

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Description(required)

This PR addresses a concern raised in Discord:

The Sessions page mentions:

For larger applications or when more robust session management is required, SolidStart also supports storing session data in a database.

But then it mentions

SolidStart does not automatically handle interactions with a database; you need to implement this yourself.

So how exactly does SolidStart support storing sessions in a database of this has to be implemented by the user?

This bit is unclear, and doesn't link to anywhere that may have an example.

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