Fix /active_record_validations.md document, `:save` for `on:` validation helper was never available

According to the guide( http://guides.rubyonrails.org/active_record_validations.html ), ":save" value for the "on:" validation helper was available like below

```ruby
    validates :name, presence: true, on: :save
```

but this was never available according to the implementation of the
valid? method, which is below

```ruby
    # 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.