Fixed #12715 -- Clarified the docs on the usage of database routers,...

The easiest way to use multiple databases is to set up a database

routing scheme. The default routing scheme ensures that objects remain

'sticky' to their original database (i.e., an object retrieved from

the ``foo`` database will be saved on the same database). However, you

can implement more interesting behaviors by defining a different

routing scheme.

the ``foo`` database will be saved on the same database). The default

routing scheme ensures that if a database isn't specified, all queries

fall back to the ``default`` database.

You don't have to do anything to activate the default routing scheme

-- it is provided 'out of the box' on every Django project. However,

if you want to implement more interesting database allocation

behaviors, you can define and install your own database routers.

Database routers

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