Add documentation for add_flash_types [ci skip]

Document a little bit the ActionController::Flash::add_flash_types
class method.
Merge pull request #11394 from robin850/patch-8

Add documentation for add_flash_types [ci skip]
 #11381: Ignore config.eager_load=true for rake

Closes #11381
Merge pull request #11389 from jetthoughts/11381_fix_hit_database_on_precompile

 #11381: Ignore config.eager_load=true for rake
Only raise DelegationError if it's is the source of the exception

This fixes situations where nested NoMethodError exceptions are masked
by delegations. This would cause confusion especially where there was a
problem in the Rails booting process because of a delegation in the
routes reloading code.

Fixes #10559
Add missing require
Fix a grammatical error/typo in Active Record Associations guide.
Merge pull request #11400 from dpdawson/master

Fix a grammatical error/typo in Active Record Associations guide. [ci skip]
Remove code_statistics so it can be required as a gem
Stub removal of rake lines LOC in ActiveRecord/ActionPack
Extracting the tools/profile code to the code_metrics gem
Re-enable stats tasks through the code_metrics gem
Re-enable rake lines tasks for actionpack and activerecord
CodeMetrics is a dependency of railties