DOC Document using symfony/validator logic

[GuySartorelli]

Issue

• Use symfony/validator for as much validation logic as we can throw at it. .github#193

[emteknetnz]

Suggested change

	You can also enforce that passwords are not repeated by setting the [`EntropyPasswordValidator.historic_count`](api:SilverStripe\Security\Validation\EntropyPasswordValidator->historic_count) configuration property:
	```yml
	SilverStripe\Security\Validation\EntropyPasswordValidator:
	historic_count: 6
	You can also enforce that passwords are not repeated by setting the [`EntropyPasswordValidator.historic_count`](api:SilverStripe\Security\Validation\PasswordValidator->historic_count) configuration property:
	```yml
	SilverStripe\Security\Validation\PasswordValidator:
	historic_count: 6

Configuration is defined on PasswordValidator, and also works if it password validator is changed to RulesPasswordValidator or a custom password validator

I assume this works if we set it on the abstract class?

[GuySartorelli]

It should work if set on the abstract class, yeah. Though IMO it makes more sense to set it on whichever validator is in use - i.e. you set the validation you want for the validator you want to use.

Given the sections are currently split up to discuss the individual validators, I'd like to keep the config being set for those validators - I think it'd read weirdly in a section about one validator you're setting config on its parent class.

All that said, I don't feel strongly so if you still want it changed I can change it.

[GuySartorelli]

Fixed lint failure