Explain "databases" in the list of explanations for the mongo config file.

[danwdart]

It's only mentioned later in the migration guide, but I think databases deserves a mention and explanation in the table of mongodb.x.json fields.

[danwdart]

In https://github.com/dadi/docs/edit/master/docs/api/4.3.md
Line 5349 has an example of "databases"
however line 5333 is the last line of the table, and the table doesn't include "databases" as something to explain.