Add note about asset pipeline and CDNs.

Fixes #2468.
Further explain the CDN and caching situation.
Properly deprecate ConnectionHandler#connection_pools

Rather than just changing it and hoping for the best.

Requested by @jeremy:

https://github.com/rails/rails/commit/ba1544d71628abff2777c9c514142d7e9a159111#commitcomment-2106059
fix warnings
Remove not used load hooks for active_record_config

These were removed with ActiveRecord::Model in
https://github.com/rails/rails/commit/9e4c41c903e8e58721f2c41776a8c60ddba7a0a9#L15L156
Deprecate Hash#diff.

It's no longer used in Rails any more.

See https://github.com/rails/rails/pull/8142\#issuecomment-10227297 for more
Merge pull request #8158 from steveklabnik/deprecate_diff

Deprecate Hash#diff.
change rdoc syntax for adecuate render [ci-skip]
correct link syntax [ci-skip]
Gracefully handle upgrading apps with mass assigment configs

Most apps upgrading from 3.x will have options for mass assigment in
their application.rb and environments/*.rb config files. Rather than
just raising a NoMethodError when copying the config, this commit
adds a warning message until either the protected_attributes gem
is installed or the relevant config options are removed.
update syntax and add note about prompt use of HttpHelpers [ci-skip]
Use get in the redirection doc examples

get is the most common usage, and match without an explicit verb
was disallowed in 56cdc81c08b1847c5c1f699810a8c3b9ac3715a6.

[ci skip]
Trivial documentation fix for ActiveModel::Naming comment.
Merge pull request #8162 from fimmtiu/master

Trivial documentation fix for ActiveModel::Naming comment [ci skip]
Update guides/source/rails_application_templates.md

Update function signatures for `#gem` and `#generate`.
Update guides/source/rails_application_templates.md

Add `#environment`
Keep the code related to serialization in Serialization module.

We should not need any `serialized_attributes` checks outside `ActiveRecord::AttributeMethods::Serialization` module.
Merge pull request #8164 from nikitug/keep_serialization_in_its_module

Keep the code related to serialization in Serialization module.
Removing warning : assigned but unused variable
Do not create useless database transaction when building `has_one` association.