ActiveModel::Errors

To be able to return type of validator, one can now call `details`
on Errors instance:

```ruby
class User < ActiveRecord::Base
  validates :name, presence: true
end
```

```ruby
user = User.new; user.valid?; user.errors.details
=> {name: [{error: :blank}]}
```

- Changed `IN` to `ON` in all note sentences in guides.

References #18148.

Splitting on whitespace makes more sense in the context of counting words in an
essay.

As pointed out in the ActiveModel::Validations::HelperMethods #validates_format_of documentation.

in the database. Closes #16304 [ci skip]

[ci skip]

Closes #9131.

Closes #9131.

Person called GoodnessValidator before it was defined.
This change will compile the example correctly.

Fixes: #13108
[ci skip]

Removed ```The `size` helper is an alias for `length`.``` line. If you use this "nonexist" helper, you will get an error message like this: 

```
ArgumentError: Unknown validator: 'SizeValidator'
...
```

Maybe wanted to mean ```validates_size_of``` helper as an alias for ```validates_length_of``` helper.

A small mistake found in the line of ```The default error is "can't be empty"``` for ```:presence``` helper. ```empty``` word changed to ```blank```.

According to the guide, ":save" value for the "on:" validation helper
was available like below

    validates :name, presence: true, on: :save

but this was never available according to the implementation of the
valid? method, which is below

    # Runs all the validations within the specified context. Returns
+true+ if
    # no errors are found, +false+ otherwise.
    #
    # If the argument is +false+ (default is +nil+), the context is set
to <tt>:create</tt> if
    # <tt>new_record?</tt> is +true+, and to <tt>:update</tt> if it is
not.
    #
    # Validations with no <tt>:on</tt> option will run no matter the
context. Validations with
    # some <tt>:on</tt> option will only run in the specified context.
    def valid?(context = nil)
      context ||= (new_record? ? :create : :update)
      output = super(context)
      errors.empty? && output
    end

So the documentation was always wrong since the PR proposed by
@neerajdotname  ( #10287 ) was rejected.

Closes #11498.

The capital O was awkward and the validation wouldn't read "right" to the user.

Added instructions of latest bundler install + modified the on: :save option description in validations