[ci skip] Fix documentation for @macro and reflection types

Since `@macro` doesn't exist anymore and these reflections are no
longer AssociationReflections but their own types of reflections
based on macro I updated the documentation to match the changes I
made in #16089 and #16198. An `AssociationReflection` that had a
`@macro` of `:has_many` now is a `HasManyReflection`
Merge pull request #16315 from eileencodes/fix-documentation-that-mentions-macro-instance-var

Fix documentation for `@macro` and reflection types
Fix example code of `EachValidator` [ci skip]

We have to specify the `:title` option to really use the
`TitleValidator` defined above.
Merge pull request #16319 from Domon/fix-each-validator-example

Fix example code of EachValidator [ci skip]
Add where condition so select_one returns the correct record.
Merge pull request #16318 from zuhao/refactor_activerecord_adapter_test

make `adapter_test` run order independently.
Merge pull request #16295 from kenn/no_stacktrace_drop_task

No verbose backtrace by db:drop when database does not exist.
Merge pull request #16297 from calebthompson/extract-iterator-method

Extract iterator method in AR::SchemaDumper
Added nodoc to change_table
Merge pull request #16303 from rajcybage/removing_master

remove empty unused method
docs, clarify attribute query methods on numeric columns. Closes #16246.

[ci skip]
Fix typo and remove code block since present is not a method.
Transactions refactoring

Add a transaction manager per connection, so it can controls the
connection responsibilities.

Delegate transaction methods to transaction_manager
savepoint_name should return nil for non-savepoint transactions

Also add test to assets the savepoint name
Merge pull request #16284 from arthurnn/transactions

Transactions refactoring
Extract the transaction class to a local variable
Merge pull request #16305 from tomkadwill/documented_change_table

Added documentation for change_table
remove some caching

this caching doesn't increase performance, but does increase complexity.
remove it for now and find better ways to speed up this code.
remove the mounted? method

we know the routes should not be "optimized" when mounting an
application
remove blank lines in the start of the ActiveRecord files