The documentation previously claimed that `db/schema.rb` was "the
authoritative source for your database schema" while simultaneously
also acknowledging that the file is generated. These two statements are
incongruous and the guides accurately call out that many database
constructs are unsupported by `schema.rb`. This change updates the
comment at the top of `schema.rb` to remove the assertion that the file
is authoritative.

The documentation also previously referred vaguely to "issues" when
re-running old migrations. This has been updated slightly to hint at the
types of problems that one can encounter with old migrations.

In sum, this change attempts to more accurately capture the pros, cons,
and shortcomings of the two schema formats in the guides and in the
comment at the top of `schema.rb`.

[Derek Prior & Sean Griffin]
Co-authored-by: NSean Griffin <sean@seantheprogrammer.com>