Document that the default for `rails server -b` has changed

Fixes #16578
Merge pull request #16984 from chancancode/fix_16578

Document that the default for `rails server -b` has changed
Fix assertions in AR::TestCase::AttributeMethodsTest

This test has always been green because it uses "assert" and the first
argument is an truthy class/object.
Merge pull request #16998 from yuki24/fix-false-assertion

Fix false assertions in AR::TestCase::AttributeMethodsTest