'Reversible' ->active_record_migrations [ci skip]

Reversible method documentation moved from migrations.md  to active_record_migrations.md
Rename remaining :in / :at to :wait / :wait_until
[ci skip] Fix typo in rails guide for routing
get rid of shadowing warning when running tests AR and railtie tests.

Warning looked like this:

```
/Users/senny/Projects/rails/activerecord/lib/active_record/associations/association_scope.rb:142: warning: shadowing outer local variable - reflection
```
build fix, we have to reset the RACK_ENV as well.

This is a follow-up to ded17a4 to get the build passing on Travis.
As Travis has `ENV["RACK_ENV"]` set to `test` we need to reset that as well.
Enums shouldn't ruin people's anniversaries

Added a few more methods on Module/Class to the dangerous class methods
blacklist. (Technically, allocate and new are already protected currently because
we happen to redefine them in the current implantation.)

Closes #16792
Merge pull request #16799 from rramsden/fix-typo

[ci skip] Fix typo in rails guide for routing
Follup to PR #16762

Remove chain from parameters, it's no longer needed since chain and i
are being passed via next_reflection

Change name of `reflection` to `owner_reflection` because of shadow
variable warning. The last reflection will always be the owner.
Merge pull request #16797 from eileencodes/refactor-add_constraints-followup-to-16762

Followup to PR #16762
Always add lambda to scope chain to eliminate branch in eval_scope

We convert all other scopes to lambda's so it makes sense that we should
always returns a lambda on a ThroughReflection as well. This eliminates
the need to check if the scope is a Relation.
Fix a warning
Merge pull request #16801 from eileencodes/refactor-scope_chain-on-through-refelction-to-eliminate-branch-in-eval_scope

Always add lambda to scope chain to eliminate branch in eval_scope
Skip StatementCache for eager loaded associations (Fixes #16761)

Eagerly loaded collection and singular associations are ignored by the StatementCache, which causes errors when the queries they generate reference columns that were not eagerly loaded.

This commit skips the creation of the StatementCache as a fix for these scenarios.
Remove example file

This documentation should be in the guides.

Closes #16691
Document that method objects are not `duplicable?`
Time#change throws exception with an out-of-range :usec

https://github.com/rails/rails/commit/98b46bf5e201307cae56ee14bf41363a539779c5
did not properly handled out-of-range `:usec`s.

Passing a `:usec` that's out of range now throws an `ArgumentError` as it
should.

Fixes #16759.
Change "For not..." to "Do not...".
Merge pull request #16804 from kaleworsley/for_not_to_do_not

Change "For not..." to "Do not...".
Merge pull request #16803 from Peeja/methods-are-not-duplicable--update-guide

Document that method objects are not `duplicable?` [ci skip]
:scissors: and from #16803 [ci skip]