Add documentation to TransactionIsolationError [ci skip]
Remove trailing whitespaces
Revert "Remove trailing whitespaces"

This reverts commit 90c887fa7d0c454b7533e208daefc342dea4d5f3.
Here's a few updates to the validations guide. A bunch of small changes,
plus:

* 80 column formats
* replacing the explanation of the dynamic_form gem with the example
  HTML/CSS that Rails scaffolds generate.
Merge pull request #8379 from steveklabnik/validations_guide

Validations guide
Migration Guide: General fix-up

This introduces a bunch of editing/re-writing to the migrations guide.
There were a bunch of small changes, and a few larger ones:

* mysql does support transactions.
* Add a section about db/seeds.rb
* Largely re-wrote the first few sections
Reset attributes should not report changes.

When resetting an attribute, you expect it to return to the state it was
before any changes. Namely, this fixes this unexpected behavior:

~~~ruby
model.name = "Bob"
model.reset_name!
model.name_changed? #=> true
~~~
Merge pull request #8391 from rmascarenhas/reset-attribute-doesnt-change-it

Reset attributes should not report changes.
Remove not used variable warnigns
Add Mailer Callback section to Mailer guides
Merge pull request #8382 from asanghi/mailerguide

Add Mailer Callback section to Mailer guides
Cleans documentation from Helpers [ci skip]
Fixes wrong typo on FormHelper [ci skip]
update AR::Schema docs [ci skip]
Add documentation to AssetTagHelper#image_alt [ci skip]
Add documentation for Hash.from_xml [ci skip]
nodoc AS::Testing::ConstantLookup::ClassMethods [ci skip]
Cleans and removes useless 'Examples' tag [ci skip]
Add db to the list of default annotation folders
Cleans and removes 'Examples' tag [ci skip]