Expand caveat about models in migrations (rails guide)

This is an attempt to address issue #6939, where an earlier migration
added a column to the database, and a later migration uses a model and
references that column.

When both migrations were run together with `rake db:migrate` the column
information in memory still referenced the old table structure.

Running the migrations separately fixed this, as a new connection was
then established before referencing the model. Explicitly calling
`reset_column_information` is a more reliable workaround.

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