ActionView::TestCase::Behavior#rendered doc misplaced

[zzak]

This bug looks introduced in #51093, where the documentation for rendered is incorrectly being attached to _routes:
https://edgeapi.rubyonrails.org/classes/ActionView/TestCase/Behavior.html#method-i-_routes

Similar to how we handled #54041, this is due to a really difficult to fix RDoc bug.

/cc @seanpdoyle

[D-Forz]

Hi! I would like to check this out, would be possible? Would be my first time contributing to Rails 😄