Document existence of default and postgres DBs, use by client connections

[rmloveland]

For details, see cockroach#24735.

tl;dr:

• Create two new databases on startup, default and postgres
• Use default if nothing is specified in the connection string
• We need postgres because some ORMs want that

[jseldess]

Thanks for creating, @rmloveland. Just fyi, after the PR merges and this change gets included in an alpha release notes, I'd definitely flag it for docs issue creation by our script. But no harm in proactively creating it early.

[rmloveland]

@jseldess totes - I created this one b/c it looks like it's going to be in the 20180604 alpha and I wanted something to add to the Docs project

[rmloveland]

Duplicate of #3206