`Rails` constant is added by rails-html-sanitizer leading to bugs in
non-Rails apps using ActiveRecord and ActionMailer

List the Active Support Instrumentation guide in the index (as WIP)

Fix grammar in Caching with Rails docs

Fix bug in ActionMailer guide.

When setting a mailer's default from address, you have to pass a hash
with a `:from` key; you can't pass just an email address.

Add support for Reply-To field in mail_to helper

Removed unused parameter to cookie serialize method

Describe full behaviour of Active Record's attribute query methods

Clarify that query methods have a custom definition of whether a numeric value is present. [ci skip]

The way Active Record query methods handle numeric values is a special case, and is not part of Rails's standard definition of present. This update attempts to make this more clear in the docs, so that people don't expect Object#present? to return false if used on a number that is zero.

References #18148.

Remove unneeded special case to calculate size for has_many :through

All cases are properly handled in CollectionAssociation
for all subclasses of this association

[ci skip]

We were ignoring the `default_value?` escape clause in the serialized
type, which caused the default value to always be treated as changed.

Fixes #18169

The code for `TableDefinition#references` and
`SchemaStatements#add_reference` were almost identical both
structurally, and in terms of domain knowledge. This removes that
duplication into a common class, using the `Table` API as the expected
interface of its collaborator.

This isn't Seattle.rb, @senny. ;)

Fix connection leak when a thread checks in additional connections.

Remove block from super

The code in `ConnectionPool#release` assumed that a single thread only
ever holds a single connection, and thus that releasing a connection
only requires the owning thread_id.

There is a trivial counterexample to this assumption: code that checks
out additional connections from the pool in the same thread. For
instance:

    connection_1 = ActiveRecord::Base.connection
    connection_2 = ActiveRecord::Base.connection_pool.checkout
    ActiveRecord::Base.connection_pool.checkin(connection_2)
    connection_3 = ActiveRecord::Base.connection

At this point, connection_1 has been removed from the
`@reserved_connections` hash, causing a NEW connection to be returned as
connection_3 and the loss of any tracking info on connection_1. As long
as the thread in this example lives, connection_1 will be inaccessible
and un-reapable. If this block of code runs more times than the size of
the connection pool in a single thread, every subsequent connection
attempt will timeout, as all of the available connections have been
leaked.

Reverts parts of 9e457a86 and
essentially all of 4367d2f0

Here you go, @senny. 😁

Add information about "allow_destroy" requiring an ID. [ci skip]

Conflicts:
	activerecord/CHANGELOG.md

I just wasted an absurd amount of time trying to figure out why my model
wasn't being deleted even though I was setting `_destroy` to true like
the instructions said. Making the documentation a little bit clear so
that someone like me doesn't waste their time in future.

Fixing numeric attrs when set to same negative value

fix typo in nodoc [ci skip]